Electrolytic solution, electrochemical device, lithium ion secondary battery, and module

ABSTRACT

wherein R101 and R102 are the same as or different from each other and are each a hydrogen atom, a fluorine atom, or an alkyl group optionally containing a fluorine atom; and R103 is an alkyl group or an organic group containing an unsaturated carbon-carbon bond. Also disclosed is an electrochemical device and lithium ion secondary battery including the electrolyte solution, and a module including the electrochemical device or lithium ion secondary battery.

TECHNICAL FIELD

The invention relates to electrolyte solutions, electrochemical devices,lithium ion secondary batteries, and modules.

BACKGROUND ART

Current electric appliances demonstrate a tendency to have a reducedweight and a smaller size, which leads to development of electrochemicaldevices such as lithium ion secondary batteries having a high energydensity. Further, electrochemical devices such as lithium ion secondarybatteries are desired to have improved characteristics as they areapplied to more various fields. Improvement in battery characteristicswill become more and more important particularly when lithium ionsecondary batteries are put in use for automobiles.

Patent Literature 1 discloses a rechargeable lithium battery including acathode, an anode containing a carbon-containing material with a degreeof crystallinity which is greater than 0.8, and an electrolyte solutioncontaining a lithium salt and a mixture of at least two aprotic organicsolvents that includes a first solvent having a high dielectric constantand a second solvent having low viscosity. The electrolyte solutionfurther contains a soluble compound which is of the same type as atleast one of the above solvents, contains at least one unsaturated bond,and can be reduced at the anode at a potential of more than 1 V withrespect to lithium to form a passivation layer.

Patent Literature 2 discloses a charging method for initially charging anon-aqueous electrolyte secondary battery including an anode, a cathode,and an electrolyte including a plurality of non-aqueous solvents, thenon-aqueous solvents including a main solvent and one or morenon-aqueous solvents having a reduction potential higher than that ofthe main solvent; the charging method including two or more chargingsteps including a first charging step in which the potential of theanode is maintained in a potential where any one of the non-aqueoussolvents having a reduction potential higher than that of the mainsolvent is reduced and decomposed and the main solvent is not reducednor decomposed.

Patent Literature 3 discloses an electrolyte for an alkali metal ionsecondary battery including an anode of a carbonaceous material, thecarbonaceous material being capable of having alkali metal ionsreversibly incorporated therein, the electrolyte containing a solutionof fluoroethylene carbonate, an alkali metal salt dissolved therein, andpropylene carbonate.

Patent Literature 4 discloses an electrolyte solution containing acyclic carbonate (I) represented by the formula (I):

(wherein Rf¹ is a fluorine-containing ether group (Ia) or afluorine-containing alkoxyl group (Ib); and X¹, X², and X³ are each H),and an electrolyte salt (II).

Patent Literature 5 discloses an electrolyte solution, containing acyclic carbonate (I-1) represented by the formula (I-1):

(wherein R is an alkyl group having two or more carbon atoms, afluorine-containing alkyl group having two or more carbon atoms, analkoxy group, or a fluorine-containing alkoxy group, in each of which anoxygen atom may be inserted between carbon atoms); a cyclic carbonate(I-2) different from the cyclic carbonate (I-1)); and an electrolytesalt (II).

CITATION LIST Patent Literature

-   Patent Literature 1: JP H8-045545 A-   Patent Literature 2: JP 2001-325988 A-   Patent Literature 3: JP 2001-501355 T-   Patent Literature 4: JP 4591505 B-   Patent Literature 5: JP 2012-216551 A

SUMMARY OF INVENTION Technical Problem

Conventional techniques fail to have sufficient effects of achievingimproved cycle characteristics, low resistance, and a reduced amount ofgas in electrochemical devices.

The invention is made in view of the above state of the art, and aims toprovide an electrolyte solution capable of providing an electrochemicaldevice that has low resistance, excellent cycle characteristics, and areduced amount of gas generated, and an electrochemical device includingthe electrolyte solution.

Solution to Problem

The inventors found that the presence of a cyclic carbonate having aspecific structure can successfully achieve the above issues, completingthe invention.

The invention relates to an electrolyte solution containing a compound(1) represented by the following formula (1):

wherein R¹⁰¹ and R¹⁰² are the same as or different from each other andare each a hydrogen atom, a fluorine atom, or an alkyl group optionallycontaining a fluorine atom; and R¹⁰³ is an alkyl group or an organicgroup containing an unsaturated carbon-carbon bond.

R¹⁰³ in the formula (1) is preferably an organic group containing anunsaturated carbon-carbon bond.

The electrolyte solution preferably contains the compound (1) in anamount of 0.001 to 10% by mass relative to the electrolyte solution.

The electrolyte solution preferably further contains an electrolytesalt.

The invention also relates to an electrochemical device including theelectrolyte solution.

The invention also relates to a lithium ion secondary battery includingthe electrolyte solution.

The invention also relates to a module including the electrochemicaldevice or the lithium ion secondary battery.

Advantageous Effects of Invention

The electrolyte solution of the invention can provide an electrochemicaldevice having low resistance, excellent cycle characteristics, and areduced amount of gas generated. The electrochemical device of theinvention has low resistance, excellent cycle characteristics, and areduced amount of gas generated.

DESCRIPTION OF EMBODIMENTS

The invention will be specifically described hereinbelow.

The electrolyte solution of the invention contains a compound (1)represented by the following formula (1).

This structure enables the electrolyte solution of the invention toprovide an electrochemical device having low resistance, excellent cyclecharacteristics, and a reduced amount of gas generated. One compound (1)may be used alone, or two or more compounds (1) may be used incombination.

In the formula (I), R¹⁰¹ and R¹⁰² are the same as or different from eachother and are each a hydrogen atom, a fluorine atom, or an alkyl groupoptionally containing a fluorine atom.

The alkyl group for R¹⁰¹ and R¹⁰² preferably has a carbon number of 1 to10, more preferably 1 to 7, still more preferably 1 to 5.

The alkyl group may or may not contain a fluorine atom.

Examples of the alkyl group free from a fluorine atom include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, a t-butyl (t-Bu) group, a sec-butyl group, a pentyl group, anisopentyl group, a hexyl group, and a cyclohexyl group. Preferred amongthese are a methyl group, an ethyl group, a propyl group, an isopropylgroup, a butyl group, a t-butyl group, and a sec-butyl group.

Examples of the alkyl group containing a fluorine atom include atrifluoromethyl group, a 2,2,2-trifluoroethyl group, a2,2,3,3-tetrafluoropropyl group, 1, 1,1,3,3,3-hexafluoropropan-2-yl,CF₃CF₂CH₂—, HCF₂CH₂—, FCH₂—, and FCH₂CH₂—. Preferred among these are atrifluoromethyl group, a 2,2,2-trifluoroethyl group, and a2,2,3,3-tetrafluoropropyl group.

The “t-butyl group” herein means a tertiary butyl group, the “sec-butylgroup” herein means a secondary butyl group.

R¹⁰¹ and R¹⁰² are the same as or different from each other and are eachpreferably a hydrogen atom or an alkyl group, more preferably a hydrogenatom or an alkyl group free from a fluorine atom, still more preferablya hydrogen atom.

In the formula (1), R¹⁰³ is an alkyl group or an organic groupcontaining an unsaturated carbon-carbon bond. The “organic group” hereinmeans a group containing at least one carbon atom and optionallycontaining an atom other than a carbon atom, such as a hydrogen atom, anoxygen atom, a nitrogen atom, a sulfur atom, or a halogen atom (e.g.,fluorine atom, chlorine atom).

When R¹⁰³ is a group that possibly has multiple stereoisomers such ascis-trans isomers, these stereoisomers are herein treated as the same.

The alkyl group for R¹⁰³ preferably has a carbon number of 1 to 10, morepreferably 1 to 7, still more preferably 1 to 5.

This alkyl group is free from a fluorine atom. The simple expression“alkyl group” (without an expression such as “optionally containing afluorine atom”) herein means an alkyl group free from a fluorine atom.

Examples of the alkyl group free from a fluorine atom include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, a t-butyl (t-Bu) group, a sec-butyl group, a pentyl group, anisopentyl group, a hexyl group, and a cyclohexyl group. Preferred amongthese are a methyl group, an ethyl group, a propyl group, an isopropylgroup, a butyl group, a t-butyl group, and a sec-butyl group, and morepreferred are a methyl group and an ethyl group.

The organic group for R¹⁰³ contains at least one unsaturatedcarbon-carbon bond. The unsaturated carbon-carbon bond is preferably acarbon-carbon double bond (—C═C—) or a carbon-carbon triple bond(—C≡C—).

The organic group preferably has a carbon number of 2 to 10, morepreferably 2 to 7, still more preferably 2 to 5.

The organic group for R¹⁰³ is preferably a C1-C10 alkyl group containingat least one unsaturated carbon-carbon bond and optionally containingone or both of a divalent or higher hetero atom and a fluorine atom. Thealkyl group preferably has a carbon number of 1 to 8, more preferably 1to 7, still more preferably 1 to 5.

The hetero atom in the organic group is preferably divalent, trivalent,or tetravalent.

Examples of the divalent or higher hetero atom include a nitrogen atom,an oxygen atom, a sulfur atom, a phosphorus atom, and a silicon atom.

An organic group for R¹⁰³ containing a fluorine atom can provide anelectrochemical device having much lower resistance.

The organic group for R¹⁰³ is preferably a group represented by thefollowing formula (X-1):

—(R^(b1))—C≡C-L¹  (X-1)

(wherein R^(b1) is an alkylene group optionally containing an oxygenatom or an unsaturated bond between carbon atoms; and L¹ is a hydrogenatom, a fluorine atom, a C1-C7 silyl or aryl group optionally containinga fluorine atom, or a C1-C7 alkyl group optionally containing one orboth of a divalent or higher hetero atom and a fluorine atom),

a group represented by the following formula (X-2):

—(R^(b2))—CL²=CL³L⁴  (X-2)

(wherein R^(b2) is a single bond or an alkylene group optionallycontaining an oxygen atom or an unsaturated bond between carbon atoms;and L², L³, and L⁴ are the same as or different from each other and areeach a hydrogen atom, a fluorine atom, a C1-C8 silyl group optionallycontaining a fluorine atom, or a C1-C8 alkyl or aryl group optionallycontaining one or both of a divalent or higher hetero atom and afluorine atom), or

a group represented by the following formula (X-3):

—(R^(b2))-L⁵  (X-3)

(wherein R^(b2) is as defined in the above; and L⁵ is a group containingan aromatic ring).

Examples of the alkyl group for L¹ include —CF₃, —CF₂CF₃, —CH₃, and—CH₂CH₃.

The silyl group for L¹ may be a group represented by the formula:—SiR^(b10)R^(c10)R^(d10) (wherein R^(b10), R^(c10), and R^(d10) are thesame as or different from each other and are each a C1-C5 alkyl groupoptionally containing a fluorine atom).

Specific examples of L¹ include a hydrogen atom, a fluorine atom, —CH₃,—CH₂CH₃, —CF₃, —CF₂CF₃, —Si(CH₃)₂ (C₄H₉), —Si(CH₃)₃, and—Si(CH₃)₂(t-Bu).

L is preferably a hydrogen atom, a fluorine atom, —Si(CH₃)₃, —CF₃,—CF₂CF₃, a phenyl group, or a perfluorophenyl group, more preferably ahydrogen atom, a fluorine atom, or —CF₃.

The carbon number of R^(b1) is preferably 1 to 8. R^(b1) is preferably agroup represented by —(CH₂)_(n11)— (wherein n11 is an integer of 1 to8). The number n11 is preferably 1 to 5, more preferably 1 to 3.

Examples of the alkyl group and aryl group for L², L³, and L⁴ include—CF₃, —CH₃, —CF₂CF₃, a phenyl group, and a perfluorophenyl group.

The silyl group for L², L³, and L⁴ may be a group represented by theformula: —SiR^(b10)R^(c10)R^(d10) (wherein R^(b10), R^(c10), and R^(d10)are the same as or different from each other and are each a C1-C5 alkylgroup optionally containing a fluorine atom).

Specific examples of the group for L², L³, and L⁴ include a hydrogenatom, a fluorine atom, —CH₃, —CH₂CH₃, —CF₃, —CF₂H, —C₂F₃(—CF₂CF₃),—Si(CH₃)₂ (t-Bu), and —Si(CH₃)₃.

L², L³, and L⁴ are preferably independently a hydrogen atom, —CH₃, —CF₃,a fluorine atom, a phenyl group, or a perfluorophenyl group, morepreferably independently a hydrogen atom, a fluorine atom, —CF₃, —CF₂H,or —C₂F₅.

Particularly preferred is a case where L² is a hydrogen atom or afluorine atom, one of L³ and L⁴ is a hydrogen atom, and the other is—CF₃, —CF₂H, or —C₂F₅.

L⁵ is a group containing an aromatic ring. Specific examples of thegroup for L⁵ include a phenyl group and a perfluorophenyl group. Thegroup represented by the above formula (X-3) is preferably an arylgroup.

R^(b2) preferably has a carbon number of 0 to 8. R^(b2) is preferably agroup represented by —(CH₂)_(n12)— (wherein n12 is an integer of 0 to8). The number n12 is preferably 0 to 5, more preferably 1 to 3.

The organic group for R¹⁰³ is also preferably a C1-C10 alkyl groupcontaining a divalent or higher hetero atom and at least one unsaturatedcarbon-carbon bond.

Examples of the divalent or higher hetero atom include a nitrogen atom,an oxygen atom, a sulfur atom, a phosphorus atom, and a silicon atom.Preferred among these are an oxygen atom and a silicon atom.

Examples of the alkyl group containing a divalent or higher hetero atomand at least one unsaturated carbon-carbon bond include—O—CH₂—CH═CH—Si(CH₃)₂(t-Bu) and —OCH₂—CH═CH—Si(CH₃)₃.

The organic group for R¹⁰³ is preferably —CH₂—CH═CH₂, —CH₂—CF═CH₂,—CH₂—CH═CH—CF₃, —CH₂—CH═CF₂, —CH₂—CF═CF₂, —CH₂—CF═CF—CF₃,—CH₂—CH═CF—CF₃, —CH₂—CH═CH—CF₂H, —CH₂—CF═CH—CF₃, —CH₂—CF═CH—CF₂H,—CH₂—CH═CH—C₂F₅, —CH₂—CF=CH—C₂F₅, —CH₂—CH═CF—Si(CH₃)₂ (tBu),—CH₂—CF═CF—Si(CH₃)₂ (tBu), —CH₂—C≡C—Si(CH₃)₂(tBu), —CH₂—C≡C-TMS,—CH₂—C≡C≡CF₃, —CH₂—C≡CH, —CH₂—C≡C—F, a phenyl group, or aperfluorophenyl group, more preferably —CH₂—CH═CH₂, —CH₂—C≡CH,—CH₂—CF═CH₂, —CH₂—CH═CH—CF₃, —CH₂—CH═CH—CF₂H, —CH₂—CF═CH—CF₃,—CH₂—CF═CH—CF₂H, —CH₂—CH═CH—C₂F₅, —CH₂—CF=CH—C₂F₅, —CH₂—C≡C—F, or—CH₂—C≡C≡CF₃, sill more preferably —CH₂—CH═CH₂, —CH₂—C≡CH, —CH₂—CF═CH₂,—CH₂—CH═CH—CF₃, or —CH₂—CH═CH—C₂F₅, particularly preferably —CH₂—CH═CH₂,—CH₂—C≡CH, —CH₂—CF═CH₂, or —CH₂—CH═CH—CF₃. The group -TMS means atrimethylsilyl group.

R¹⁰³ is preferably an organic group containing an unsaturatedcarbon-carbon bond, more preferably a group represented by the formula(X-1) or the formula (X-2), still more preferably —CH₂—CH═CH₂,—CH₂—C≡CH, —CH₂—CF═CH₂, —CH₂—CH═CH—CF₃, —CH₂—CH═CH—CF₂H, —CH₂—CF═CH—CF₃,—CH₂—CF═CH—CF₂H, —CH₂—CH═CH—C₂F₅, —CH₂—CF=CH—C₂F₅, —CH₂—C≡C—F, or—CH₂—C≡C≡CF₃, particularly preferably —CH₂—CH═CH₂, —CH₂—C≡CH,—CH₂—CF═CH₂, —CH₂—CH═CH—CF₃, or —CH₂—CH═CH—C₂F₅, most preferably—CH₂—CH═CH₂, —CH₂—C≡CH, —CH₂—CF═CH₂, or —CH₂—CH═CH—CF₃.

Specific examples of the compound (1) include compounds represented bythe following formulas.

In the formulas, “Me” is —CH₃, “Et” is —CH₂CH₃, “n-Pr” is —CH₂CH₂CH₃,“i-Pr” is —CH(CH₃)₂, “n-Bu” is —CH₂CH₂CH₂CH₃, “sec-Bu” is —CH(CH₃)CH₂CH₃, and “t-Bu” is —C(CH₃)₃.

The compound (1) is preferably any of the compounds represented by thefollowing formulas:

more preferably any of the compounds represented by the followingformulas:

The compound (1) can be suitably produced by a production methodincluding a step of reacting an unsaturated cyclic carbonate representedby the following formula (11):

(wherein R¹⁰¹ and R¹⁰² are as defined in the above) and an alcoholrepresented by the following formula (12):

R¹⁰³—OH

(wherein R¹⁰³ is as defined in the above) or an alkoxide thereof in thepresence of a base or reacting the unsaturated cyclic carbonate and thealkoxide.

Specific examples of the unsaturated cyclic carbonate represented by theformula (11) include compounds represented by the following formulas.

Specific examples of the alcohol represented by the formula (12) includeR¹⁰⁴—OH (wherein R¹⁰⁴ is an alkyl group), CH—C≡CH₂—OH, CH₂═CH—CH₂—OH,CH═CFCH₂—OH, CF₃—CH═CH—CH₂—OH, Si(CH₃)₂ (t-Bu)—CH═CH—CH₂—OH,CF₂═CF—CH₂—OH, CF₂═CH—CH₂—OH, CF₃—CF═CF—CH₂—OH, CF₃—CF═CH—CH₂—OH,Si(CH₃)₂(t-Bu)-CF═CH—CH₂—OH, Si(CH₃)₂(t-Bu)—CF═CF—CH₂—OH,TMS—C≡C—CH₂—OH, CF₃—C≡C—CH₂—OH, CF—C≡CH₂—OH, Si(CH₃) 2(t-Bu)—C≡C—CH₂—OH, a phenol, and a pentafluorophenol.

Preferred among these is at least one selected from the group consistingof CH—C≡CH₂—OH, CH₂═CH—CH₂—OH, CH═CFCH₂—OH, CF₃—CH═CH—CH₂—OH, and aphenol.

Examples of an alkoxide of the alcohol represented by the formula (12)include an ammonium alkoxide and a metal alkoxide of the alcohol. Themetal alkoxide may be either an alkoxide of a monovalent metal or analkoxide of a divalent metal. Examples thereof include metal alkoxidesof a metal such as lithium, sodium, potassium, magnesium, calcium, orcesium.

The production method includes a step of reacting an unsaturated cycliccarbonate represented by the formula (11) and an alcohol represented bythe formula (12) or an alkoxide thereof in the presence of a base orreacting the unsaturated cyclic carbonate and the alkoxide (hereinafter,referred to as a “reaction step”).

The base may be any base and may be either an inorganic base or anorganic base.

The base may be either a weak base or a strong base, and is preferably astrong base. A strong base enables smooth progress of the reaction step.

In a reaction of the unsaturated cyclic carbonate represented by theformula (11) and an alkoxide of the alcohol represented by the formula(12), the reaction proceeds even without the base. Thus, the reactionmay be performed either in the presence or absence of the base.

The base is preferably at least one selected from the group consistingof a hydride of an alkali metal or alkaline-earth metal, a hydroxide ofan alkali metal or alkaline-earth metal, a carbonate compound of analkali metal or an alkaline-earth metal, a hydrogen carbonate compoundof an alkali metal, an alkoxide of an alkali metal or alkaline-earthmetal, an amide of an alkali metal or alkaline-earth metal, guanidine,and amines.

Examples of the hydride include NaH, LiH, and CaH₂.

Examples of the hydroxide include LiOH, KOH, NaOH, Ca(OH)₂, Ba(OH)₂,Mg(OH)₂, Cu(OH)₂, Al(OH)₃, and Fe(OH)₃.

Examples of the carbonate compound include K₂CO₃, Na₂CO₃, CaCO₃, andCsCO₃.

Examples of the hydrogen carbonate compound include NaHCO₃ and KHCO₃.

Examples of the alkoxide include a potassium methoxide, a potassiumethoxide, a potassium propoxide, a potassium butoxide, a sodiummethoxide, a sodium ethoxide, a sodium propoxide, and a sodium butoxide.

Examples of the amine include triethyl amine, diisopropylethylamine,tributyl amine, ethyl diisopropyl amine, pyridine, imidazole, N-methylimidazole, N,N′-dimethyl amino pyridine, picoline,1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,4-diazabicyclo[2.2.2]octane(DABCO), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).

Examples of the amide include sodium amide and lithium diisopropylamide.

The base is preferably at least one selected from the group consistingof NaH, LiH, guanidine, and amines, more preferably at least oneselected from the group consisting of NaH and amines.

Also, a base such as butyl lithium or N-methyl morpholine may beemployed.

The amount of the base used in the reaction step is preferably 0.9 to1.1 equivalents based on the amount of the unsaturated cyclic carbonaterepresented by the formula (11).

The base may be used in an excessive amount. The amount of the base ispreferably 1 to 25 mol %, more preferably 1 to 10 mol %, still morepreferably 1 to 6 mol %, based on the amount of the unsaturated cycliccarbonate represented by the formula (11).

In the reaction step, the amount of the alcohol represented by theformula (12) or an alkoxide thereof is preferably 0.9 to 1.1 equivalentsbased on the unsaturated cyclic carbonate represented by the formula(11).

The alcohol represented by the formula (12) or an alkoxide thereof maybe used in an excessive amount. The amount of the alcohol or an alkoxidethereof is preferably 1 to 20 equivalents, more preferably 1.1 to 10equivalents, based on the unsaturated cyclic carbonate represented bythe formula (11).

The reaction step may be performed in the presence of a solvent(excepting the alcohol represented by the formula (12)). The solvent ispreferably an aprotic solvent, and examples thereof includetetrahydrofuran, monoglyme, diethyl alkoxyalkylene, and acetonitrile.

In the production method, the alcohol represented by the formula (12)may be used as a solvent. Thus, the reaction can be performed withoutthe solvent (excepting the alcohol represented by the formula (12)).

The temperature for the reaction step is preferably 20° C. or lower,more preferably 5° C. or lower, while preferably 0° C. or higher.

The reaction duration is not limited and may be 60 to 240 minutes, forexample.

The mixed product obtained through the above reaction step may beseparated into the respective components by a known method such ascoagulation and crystallization.

The electrolyte solution of the invention may contain the compound (1)in an amount of 0.001 to 15% by mass, preferably 0.001 to 10% by mass,relative to the electrolyte solution. The compound (1) in an amountwithin the above range can provide an electrochemical device having muchlower resistance, much better cycle characteristics, and a much morereduced amount of gas generated. The amount of the compound (1) is morepreferably 0.01% by mass or more, still more preferably 0.10% by mass ormore, while more preferably 5.0% by mass or less, still more preferably2.0% by mass or less.

The electrolyte solution of the invention preferably contains a solventother than the compound (1).

The solvent preferably includes at least one selected from the groupconsisting of a carbonate and a carboxylate.

The carbonate may be either a cyclic carbonate or an acyclic carbonate.

The cyclic carbonate may be either a non-fluorinated cyclic carbonate ora fluorinated cyclic carbonate.

An example of the non-fluorinated cyclic carbonate is a non-fluorinatedsaturated cyclic carbonate. Preferred is a non-fluorinated saturatedalkylene carbonate containing a C2-C6 alkylene group, more preferred isa non-fluorinated saturated alkylene carbonate containing a C2-C4alkylene group.

In order to give high permittivity and suitable viscosity, thenon-fluorinated saturated cyclic carbonate preferably includes at leastone selected from the group consisting of ethylene carbonate, propylenecarbonate, cis-2,3-pentylene carbonate, cis-2,3-butylene carbonate,2,3-pentylene carbonate, 2,3-butylene carbonate, 1,2-pentylenecarbonate, 1,2-butylene carbonate, and butylene carbonate.

One non-fluorinated saturated cyclic carbonate may be used alone, or twoor more thereof may be used in any combination at any ratio.

The non-fluorinated saturated cyclic carbonate, when contained, ispreferably present in an amount of 5 to 90% by volume, more preferably10 to 60% by volume, still more preferably 15 to 45% by volume, relativeto the solvent.

The fluorinated cyclic carbonate is a cyclic carbonate containing afluorine atom. A solvent containing a fluorinated cyclic carbonate cansuitably be used at high voltage.

The term “high voltage” herein means a voltage of 4.2 V or higher. Theupper limit of the “high voltage” is preferably 4.9 V.

The fluorinated cyclic carbonate may be either a fluorinated saturatedcyclic carbonate or a fluorinated unsaturated cyclic carbonate.

The fluorinated saturated cyclic carbonate is a saturated cycliccarbonate containing a fluorine atom. Specific examples thereof includea compound represented by the following formula (A):

(wherein X¹ to X⁴ are the same as or different from each other, and areeach —H, —CH₃, —C₂H₅, —F, a fluorinated alkyl group optionallycontaining an ether bond, or a fluorinated alkoxy group optionallycontaining an ether bond; at least one selected from X¹ to X⁴ is —F, afluorinated alkyl group optionally containing an ether bond, or afluorinated alkoxy group optionally containing an ether bond). Examplesof the fluorinated alkyl group include —CF₃, —CF₂H, and —CH₂F.

The presence of the fluorinated saturated cyclic carbonate in theelectrolyte solution of the invention when applied to a high-voltagelithium ion secondary battery, for example, can improve the oxidationresistance of the electrolyte solution, resulting in stable andexcellent charge and discharge characteristics.

The term “ether bond” herein means a bond represented by —O—.

In order to give a good permittivity and oxidation resistance, one ortwo of X¹ to X⁴ is/are each preferably —F, a fluorinated alkyl groupoptionally containing an ether bond, or a fluorinated alkoxy groupoptionally containing an ether bond.

In anticipation of a decrease in viscosity at low temperature, anincrease in flash point, and improvement in solubility of an electrolytesalt, X¹ to X⁴ are each preferably —H, —F, a fluorinated alkyl group(a), a fluorinated alkyl group (b) containing an ether bond, or afluorinated alkoxy group (c).

The fluorinated alkyl group (a) is a group obtainable by replacing atleast one hydrogen atom of an alkyl group by a fluorine atom. Thefluorinated alkyl group (a) preferably has a carbon number of 1 to 20,more preferably 1 to 17, still more preferably 1 to 7, particularlypreferably 1 to 5.

Too large a carbon number may cause poor low-temperature characteristicsand low solubility of an electrolyte salt. Too small a carbon number maycause low solubility of an electrolyte salt, low discharge efficiency,and increased viscosity, for example.

Examples of the fluorinated alkyl group (a) having a carbon number of 1include CFH₂—, CF₂H—, and CF₃—. In order to give good high-temperaturestorage characteristics, particularly preferred is CF₂H— or CF₃—. Mostpreferred is CF₃—.

In order to give good solubility of an electrolyte salt, preferredexamples of the fluorinated alkyl group (a) having a carbon number of 2or greater include fluorinated alkyl groups represented by the followingformula (a-1):

R¹—R²—  (a-1)

wherein R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom; R² is a C1-C3 alkylene groupoptionally containing a fluorine atom; and at least one selected from R¹and R² contains a fluorine atom.

R¹ and R² each may further contain an atom other than carbon, hydrogen,and fluorine atoms.

R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom. R¹ is preferably a C1-C16 linearor branched alkyl group. The carbon number of R¹ is more preferably 1 to6, still more preferably 1 to 3.

Specifically, for example, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CH₃CH₂CH₂CH₂—, andgroups represented by the following formulae:

may be mentioned as linear or branched alkyl groups for R¹.

Examples of R¹ which is a linear alkyl group containing a fluorine atominclude CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—,CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂—,FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CF₂CH₂CF₂CH₂—,CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, HCFClCF₂CH₂—, HCF₂CFClCH₂—,HCF₂CFClCF₂CFClCH₂—, and HCFClCF₂CFClCF₂CH₂—.

Examples of R¹ which is a branched alkyl group containing a fluorineatom include those represented by the following formulae.

The presence of a branch such as CH₃— or CF₃— may easily cause highviscosity. Thus, the number of such branches is more preferably small(one) or zero.

R² is a C1-C3 alkylene group optionally containing a fluorine atom. R²may be either linear or branched. Examples of a minimum structural unitconstituting such a linear or branched alkylene group are shown below.R² is constituted by one or combination of these units.

(i) Linear Minimum Structural Units

-   -   —CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

R² which is a linear group consists only of any of the above linearminimum structural units, and is preferably —CH₂—, —CH₂CH₂—, or CF₂—. Inorder to further improve the solubility of an electrolyte salt, —CH₂— or—CH₂CH₂— is more preferred.

R² which is a branched group includes at least one of the above branchedminimum structural units. A preferred example thereof is a grouprepresented by —(CX^(a)X^(b))— (wherein X^(a) is H, F, CH₃, or CF₃;X^(b) is CH₃ or CF₃; when X^(b) is CF₃, X^(a) is H or CH₃). Such a groupcan much further improve the solubility of an electrolyte salt.

For example, CF₃CF₂—, HCF₂CF₂—, H₂CFCF₂—, CH₃CF₂—, CF₃CHF—, CH₃CF₂—,CF₃CF₂CF₂—, HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those representedby the following formulae:

may be mentioned as preferred examples of the fluorinated alkyl group(a).

The fluorinated alkyl group (b) containing an ether bond is a groupobtainable by replacing at least one hydrogen atom of an alkyl groupcontaining an ether bond by a fluorine atom. The fluorinated alkyl group(b) containing an ether bond preferably has a carbon number of 2 to 17.Too large a carbon number may cause high viscosity of the fluorinatedsaturated cyclic carbonate. This may also cause the presence of manyfluorine-containing groups, resulting in poor solubility of anelectrolyte salt due to reduction in permittivity, and poor miscibilitywith other solvents. Accordingly, the carbon number of the fluorinatedalkyl group (b) containing an ether bond is preferably 2 to 10, morepreferably 2 to 7.

The alkylene group which constitutes the ether moiety of the fluorinatedalkyl group (b) containing an ether bond is a linear or branchedalkylene group. Examples of a minimum structural unit constituting sucha linear or branched alkylene group are shown below.

(i) Linear Minimum Structural Units

-   -   —CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

The alkylene group may be constituted by one of these minimum structuralunits, or may be constituted by multiple linear units (i), by multiplebranched units (ii), or by a combination of a linear unit (i) and abranched unit (ii). Preferred examples will be mentioned in detaillater.

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

A still more preferred example of the fluorinated alkyl group (b)containing an ether bond is a group represented by the following formula(b-1):

R³—(OR⁴)_(n1)—  (b-1)

wherein R³ is preferably a C1-C6 alkyl group optionally containing afluorine atom; R⁴ is preferably a C1-C4 alkylene group optionallycontaining a fluorine atom; n1 is an integer of 1 to 3; and at least oneselected from R³ and R⁴ contains a fluorine atom.

Examples of R³ and R⁴ include the following groups, and any appropriatecombination of these groups can provide the fluorinated alkyl group (b)containing an ether bond represented by the formula (b-1). Still, thegroups are not limited thereto.

(1) R³ is preferably an alkyl group represented by the formula: X^(c)₃C—(R)_(n2)—, wherein three X^(c)s are the same as or different fromeach other, and are each H or F; R⁵ is a C1-C5 alkylene group optionallycontaining a fluorine atom; and n2 is 0 or 1.

When n2 is 0, R³ may be CH₃—, CF₃—, HCF₂—, or H₂CF—, for example.

When n2 is 1, specific examples of R³ which is a linear group includeCF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—, CF₃CH₂CF₂—,CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CHCH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂CH₂—,FCH₂CF₂—, FCH₂CF₂CH₂—, CH₃CF₂—, CH₃CH₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CH₂CH₂—, CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CH₂CH₂CH₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂CH₂—, and CH₃CF₂CH₂CF₂CH₂CH₂—

When n2 is 1, those represented by the following formulae:

may be mentioned as examples of R³ which is a branched group.

The presence of a branch such as CH₃— or CF₃— may easily cause highviscosity. Thus, R³ is more preferably a linear group.

(2) In —(OR⁴)_(n1)— of the formula (b-1), n1 is an integer of 1 to 3,preferably 1 or 2. When n1 is 2 or 3, R⁴s may be the same as ordifferent from each other.

Preferred specific examples of R⁴ include the following linear orbranched groups.

Examples of the linear groups include —CH₂—, —CHF—, —CF₂—, —CH₂CH₂—,—CF₂CH₂—, —CF₂CF₂—, —CH₂CF₂—, —CH₂CH₂CH₂—, —CH₂CH₂CF₂—, —CH₂CF₂CH₂—,—CH₂CF₂CF₂—, —CF₂CH₂CH₂—, —CF₂CF₂CH₂—, —CF₂CH₂CF₂—, and —CF₂CF₂CF₂—.

Those represented by the following formulae:

may be mentioned as examples of the branched groups.

The fluorinated alkoxy group (c) is a group obtainable by replacing atleast one hydrogen atom of an alkoxy group by a fluorine atom. Thefluorinated alkoxy group (c) preferably has a carbon number of 1 to 17,more preferably 1 to 6.

The fluorinated alkoxy group (c) is particularly preferably afluorinated alkoxy group represented by X^(d) ₃C—(R⁶)_(n3)—O—, whereinthree X^(d)s are the same as or different from each other, and are eachH or F; R⁶ is preferably a C1-C5 alkylene group optionally containing afluorine atom; n3 is 0 or 1; and any of the three X^(d)s contain afluorine atom.

Specific examples of the fluorinated alkoxy group (c) includefluorinated alkoxy groups in which an oxygen atom binds to an end of analkyl group mentioned as an example for R¹ in the formula (a-1).

The fluorinated alkyl group (a), the fluorinated alkyl group (b)containing an ether bond, and the fluorinated alkoxy group (c) in thefluorinated saturated cyclic carbonate each preferably have a fluorinecontent of 10% by mass or more. Too less a fluorine content may cause afailure in sufficiently achieving an effect of reducing the viscosity atlow temperature and an effect of increasing the flash point. Thus, thefluorine content is more preferably 12% by mass or more, still morepreferably 15% by mass or more. The upper limit thereof is usually 76%by mass.

The fluorine content of each of the fluorinated alkyl group (a), thefluorinated alkyl group (b) containing an ether bond, and thefluorinated alkoxy group (c) is a value calculated based on thecorresponding structural formula by the following formula:

{(Number of fluorine atoms×19)/(Formula weight of group)}×100(%).

In order to give good permittivity and oxidation resistance, thefluorine content in the whole fluorinated saturated cyclic carbonate ispreferably 10% by mass or more, more preferably 15% by mass or more. Theupper limit thereof is usually 76% by mass.

The fluorine content in the fluorinated saturated cyclic carbonate is avalue calculated based on the structural formula of the fluorinatedsaturated cyclic carbonate by the following formula:

{(Number of fluorine atoms×19)/(Molecular weight of fluorinatedsaturated cyclic carbonate)}×100(%).

Specific examples of the fluorinated saturated cyclic carbonate includethe following.

Specific examples of the fluorinated saturated cyclic carbonate in whichat least one selected from X¹ to X, is —F include those represented bythe following formulae.

These compounds have a high withstand voltage and give good solubilityof an electrolyte salt.

Alternatively, those represented by the following formulae:

may also be used.

Those represented by the following formulae:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate in which at least one selected from X¹ to X⁴ is afluorinated alkyl group (a) and the others are —H.

Those represented by the following formulae:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate in which at least one selected from X¹ to X⁴ is afluorinated alkyl group (b) containing an ether bond or a fluorinatedalkoxy group (c) and the others are —H.

In particular, the fluorinated saturated cyclic carbonate is preferablyany of the following compounds.

Examples of the fluorinated saturated cyclic carbonate also includetrans-4,5-difluoro-1,3-dioxolan-2-one,5-(1,1-difluoroethyl)-4,4-difluoro-1,3-dioxolan-2-one,4-methylene-1,3-dioxolan-2-one,4-methyl-5-trifluoromethyl-1,3-dioxolan-2-one,4-ethyl-5-fluoro-1,3-dioxolan-2-one,4-ethyl-5,5-difluoro-1,3-dioxolan-2-one,4-ethyl-4,5-difluoro-1,3-dioxolan-2-one,4-ethyl-4,5,5-trifluoro-1,3-dioxolan-2-one,4,4-difluoro-5-methyl-1,3-dioxolan-2-one,4-fluoro-5-methyl-1,3-dioxolan-2-one,4-fluoro-5-trifluoromethyl-1,3-dioxolan-2-one, and4,4-difluoro-1,3-dioxolan-2-one.

More preferred among these as the fluorinated saturated cyclic carbonateare fluoroethylene carbonate, difluoroethylene carbonate,trifluoromethylethylene carbonate, (3,3,3-trifluoropropylene carbonate),and 2,2,3,3,3-pentafluoropropylethylene carbonate.

The fluorinated unsaturated cyclic carbonate is a cyclic carbonatecontaining an unsaturated bond and a fluorine atom, and is preferably afluorinated ethylene carbonate derivative substituted with a substituentcontaining an aromatic ring or a carbon-carbon double bond. Specificexamples thereof include 4,4-difluoro-5-phenyl ethylene carbonate,4,5-difluoro-4-phenyl ethylene carbonate, 4-fluoro-5-phenyl ethylenecarbonate, 4-fluoro-5-vinyl ethylene carbonate, 4-fluoro-4-phenylethylene carbonate, 4,4-difluoro-4-vinyl ethylene carbonate,4,4-difluoro-4-allyl ethylene carbonate, 4-fluoro-4-vinyl ethylenecarbonate, 4-fluoro-4,5-diallyl ethylene carbonate, 4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4,5-divinyl ethylene carbonate, and4,5-difluoro-4,5-diallyl ethylene carbonate.

One fluorinated cyclic carbonate may be used alone or two or morethereof may be used in any combination at any ratio.

The fluorinated cyclic carbonate, when contained, is preferably presentin an amount of 5 to 90% by volume, more preferably 10 to 60% by volume,still more preferably 15 to 45% by volume, relative to the solvent.

The acyclic carbonate may be either a non-fluorinated acyclic carbonateor a fluorinated acyclic carbonate.

Examples of the non-fluorinated acyclic carbonate includehydrocarbon-based acyclic carbonates such as CH₃OCOOCH₃ (dimethylcarbonate, DMC), CH₃CH₂OCOOCH₂CH₃ (diethyl carbonate, DEC),CH₃CH₂OCOOCH₃ (ethyl methyl carbonate, EMC), CH₃OCOOCH₂CH₂CH₃ (methylpropyl carbonate), methyl butyl carbonate, ethyl propyl carbonate, ethylbutyl carbonate, dipropyl carbonate, dibutyl carbonate, methyl isopropylcarbonate, methyl-2-phenyl phenyl carbonate, phenyl-2-phenyl phenylcarbonate, trans-2,3-pentylene carbonate, trans-2,3-butylene carbonate,and ethyl phenyl carbonate. Preferred among these is at least oneselected from the group consisting of ethyl methyl carbonate, diethylcarbonate, and dimethyl carbonate.

One non-fluorinated acyclic carbonate may be used alone or two or morethereof may be used in any combination at any ratio.

The non-fluorinated acyclic carbonate, when contained, is preferablypresent in an amount of 10 to 90% by volume, more preferably 40 to 85%by volume, still more preferably 50 to 80% by volume, relative to thesolvent.

The fluorinated acyclic carbonate is an acyclic carbonate containing afluorine atom. A solvent containing a fluorinated acyclic carbonate cansuitably be used at high voltage.

An example of the fluorinated acyclic carbonate is a compoundrepresented by the following formula (B):

Rf²OCOOR⁷  (B)

wherein Rf² is a C1-C7 fluorinated alkyl group; and R⁷ is a C1-C7 alkylgroup optionally containing a fluorine atom.

Rf² is a C1-C7 fluorinated alkyl group and R⁷ is a C1-C7 alkyl groupoptionally containing a fluorine atom.

The fluorinated alkyl group is a group obtainable by replacing at leastone hydrogen atom of an alkyl group by a fluorine atom. When R⁷ is analkyl group containing a fluorine atom, it is a fluorinated alkyl group.

In order to give low viscosity, Rf² and R⁷ each preferably have a carbonnumber of 1 to 7, more preferably 1 to 2.

Too large a carbon number may cause poor low-temperature characteristicsand low solubility of an electrolyte salt. Too small a carbon number maycause low solubility of an electrolyte salt, low discharge efficiency,and increased viscosity, for example.

Examples of the fluorinated alkyl group having a carbon number of 1include CFH₂—, CF₂H—, and CF₃—. In order to give high-temperaturestorage characteristics, particularly preferred is CFH₂— or CF₃—.

In order to give good solubility of an electrolyte salt, preferredexamples of the fluorinated alkyl group having a carbon number of 2 orgreater include fluorinated alkyl groups represented by the followingformula (d-1):

R¹—R²—  (d-1)

wherein R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom; R² is a C1-C3 alkylene groupoptionally containing a fluorine atom; and at least one selected from R¹and R² contains a fluorine atom.

R¹ and R² each may further contain an atom other than carbon, hydrogen,and fluorine atoms.

R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom. R¹ is preferably a C1-C6 linearor branched alkyl group. The carbon number of R¹ is more preferably 1 to3.

Specifically, for example, CH₃—, CF₃—, CH₃CH₂—, CH₃CH₂CH₂—,CH₃CH₂CH₂CH₂—, and groups represented by the following formulae:

may be mentioned as linear or branched alkyl groups for R¹.

Examples of R¹ which is a linear alkyl group containing a fluorine atominclude CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—,CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,C F₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂—,FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CF₂CH₂CF₂CH₂—,CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, HCFClCF₂CH₂—, HCF₂CFClCH₂—,HCF₂CFClCF₂CFClCH₂—, and HCFClCF₂CFClCF₂CH₂—.

Examples of R¹ which is a branched alkyl group containing a fluorineatom include those represented by the following formulae.

The presence of a branch such as CH₃— or CF₃— may easily cause highviscosity. Thus, the number of such branches is more preferably small(one) or zero.

R² is a C1-C3 alkylene group optionally containing a fluorine atom. R²may be either linear or branched. Examples of a minimum structural unitconstituting such a linear or branched alkylene group are shown below.R² is constituted by one or combination of these units.

(i) Linear Minimum Structural Units

-   -   —CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

R² which is a linear group consists only of any of the above linearminimum structural units, and is preferably —CH₂—, —CH₂CH₂—, or —CF₂—.In order to further improve the solubility of an electrolyte salt, —CH₂—or —CH₂CH₂— is more preferred.

R² which is a branched group includes at least one of the above branchedminimum structural units. A preferred example thereof is a grouprepresented by —(CX^(a)X^(b))— (wherein X^(a) is H, F, CH₃, or CF₃;X^(b) is CH₃ or CF₃; when X^(b) is CF₃, X^(a) is H or CH₃). Such a groupcan much further improve the solubility of an electrolyte salt.

For example, CF₃CF₂—, HCF₂CF₂—, H₂CFCF₂—, CH₃CF₂—, CF₃CH₂—, CF₃CF₂CF₂—,HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those represented by thefollowing formulae:

may be specifically mentioned as preferred examples of the fluorinatedalkyl group.

The fluorinated alkyl group for Rf² and R⁷ is preferably CF₃—, CF₃CF₂—,(CF₃)₂CH—, CF₃CH₂—, C₂F₅CH₂—, CF₃CF₂CH₂—, HCF₂CF₂CH₂—, CF₃CFHCF₂CH₂—,CFH₂—, and CF₂H—. In order to give high incombustibility and good ratecharacteristics and oxidation resistance, more preferred are CF₃CH₂—,CF₃CF₂CH₂—, HCF₂CF₂CH₂—, CFH₂—, and CF₂H—.

R⁷, when it is an alkyl group free from a fluorine atom, is a C1-C7alkyl group. In order to give low viscosity, R⁷ preferably has a carbonnumber of 1 to 4, more preferably 1 to 3.

Examples of the alkyl group free from a fluorine atom include CH₃—,CH₃CH₂—, (CH₃)₂CH—, and C₃H₇—. In order to give low viscosity and goodrate characteristics, preferred are CH₃— and CH₃CH₂—.

The fluorinated acyclic carbonate preferably has a fluorine content of15 to 70% by mass. The fluorinated acyclic carbonate having a fluorinecontent within the above range can maintain the miscibility with asolvent and the solubility of a salt. The fluorine content is morepreferably 20% by mass or more, still more preferably 30% by mass ormore, particularly preferably 35% by mass or more, while more preferably60% by mass or less, still more preferably 50% by mass or less.

In the invention, the fluorine content is a value calculated based onthe structural formula of the fluorinated acyclic carbonate by thefollowing formula:

{(Number of fluorine atoms×19)/(Molecular weight of fluorinated acycliccarbonate)}×100(%).

In order to give low viscosity, the fluorinated acyclic carbonate ispreferably any of the following compounds.

The fluorinated acyclic carbonate is particularly preferably methyl2,2,2-trifluoroethyl carbonate.

One fluorinated acyclic carbonate may be used alone, or two or morethereof may be used in any combination at any ratio.

The fluorinated acyclic carbonate, when contained, is preferably presentin an amount of 10 to 90% by volume, more preferably 40 to 85′ byvolume, still more preferably 50 to 80% by volume, relative to thesolvent.

The carboxylate may be either a cyclic carboxylate or an acycliccarboxylate.

The cyclic carboxylate may be either a non-fluorinated cycliccarboxylate or a fluorinated cyclic carboxylate.

Examples of the non-fluorinated cyclic carboxylate include anon-fluorinated saturated cyclic carboxylate, and preferred is anon-fluorinated saturated cyclic carboxylate containing a C2-C4 alkylenegroup.

Specific examples of the non-fluorinated saturated cyclic carboxylatecontaining a C2-C4 alkylene group include β-propiolactone,γ-butyrolactone, ε-caprolactone, δ-valerolactone, andα-methyl-γ-butyrolactone. In order to improve the degree of dissociationof lithium ions and to improve the load characteristics, particularlypreferred among these are γ-butyrolactone and δ-valerolactone.

One non-fluorinated saturated cyclic carboxylate may be used alone ortwo or more thereof may be used in any combination at any ratio.

The non-fluorinated saturated cyclic carboxylate, when contained, ispreferably present in an amount of 0 to 90% by volume, more preferably0.001 to 90% by volume, still more preferably 1 to 60% by volume,particularly preferably 5 to 40% by volume, relative to the solvent.

The acyclic carboxylate may be either a non-fluorinated acycliccarboxylate or a fluorinated acyclic carboxylate.

Examples of the non-fluorinated acyclic carboxylate include methylacetate, ethyl acetate, propyl acetate, butyl acetate, methylpropionate, ethyl propionate, propyl propionate, butyl propionate,tert-butyl propionate, tert-butyl butyrate, sec-butyl propionate,sec-butyl butyrate, n-butyl butyrate, methyl pyrophosphate, ethylpyrophosphate, tert-butyl formate, tert-butyl acetate, sec-butylformate, sec-butyl acetate, n-hexyl pivalate, n-propyl formate, n-propylacetate, n-butyl formate, n-butyl pivalate, n-octyl pivalate, ethyl2-(dimethoxyphosphoryl)acetate, ethyl 2-(dimethylphosphoryl)acetate,ethyl 2-(diethoxyphosphoryl)acetate, ethyl 2-(diethylphosphoryl)acetate,isopropyl propionate, isopropyl acetate, ethyl formate, ethyl 2-propynyloxalate, isopropyl formate, isopropyl butyrate, isobutyl formate,isobutyl propionate, isobutyl butyrate, and isobutyl acetate.

Preferred among these are butyl acetate, methyl propionate, ethylpropionate, propyl propionate, and butyl propionate, particularlypreferred are ethyl propionate and propyl propionate.

One non-fluorinated acyclic carboxylate may be used alone or two or morethereof may be used in any combination at any ratio.

The non-fluorinated acyclic carboxylate, when contained, is preferablypresent in an amount of 0 to 90% by volume, more preferably 0.001 to 90%by volume, still more preferably 1 to 60% by volume, particularlypreferably 5 to 40% by volume, relative to the solvent.

The non-fluorinated acyclic ester is preferably present in an amount of0 to 90% by volume, more preferably 0.001 to 90% by volume, still morepreferably 1 to 60% by volume, particularly preferably 5 to 40% byvolume, relative to the solvent.

The fluorinated acyclic carboxylate is an acyclic carboxylate containinga fluorine atom. A solvent containing a fluorinated acyclic carboxylatecan be suitably used at high voltage.

In order to achieve good miscibility with other solvents and to givegood oxidation resistance, preferred examples of the fluorinated acycliccarboxylate include a fluorinated acyclic carboxylate represented by thefollowing formula:

Rf³¹ COORf³²

(wherein Rf³¹ is a C1-C4 fluorinated alkyl group; and Rf³² is a C1-C4alkyl group optionally containing a fluorine atom).

Examples of Rf³¹ include HCF₂—, CF₃—, CF₃CF₂—, HCF₂CF₂—, CH₃CF₂—,CF₃CH₂—, and CH₃—. In order to achieve good viscosity and oxidationresistance, particularly preferred among these are HCF₂—, CF₃—, CF₃CF₂—,and CF₃CH₂—.

Examples of Rf³² include CH₃—, C₂H₅—, CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—,CF₃CH₂CH₂—, CF₃CFHCF₂CH₂—, C₂F₅CH₂—, CF₂HCF₂CH₂—, C₂F₅CH₂CH₂—,CF₃CF₂CH₂—, and CF₃CF₂CF₂CH₂—. In order to achieve good miscibility withother solvents, particularly preferred among these are CH₃—, C₂H₅—,CF₃CH₂—, and CF₃CH₂CH₂—.

Specific examples of the fluorinated acyclic carboxylate include one ortwo or more of CF₃CH₂C(═O)OCH₃ (methyl 3,3,3-trifluoropropionate),HCF₂C(═O)OCH₃ (methyl difluoroacetate), HCF₂C(═O)OC₂H₅(ethyldifluoroacetate), CF₃C(═O) OCH₂CH₂CF₃, CF₃C(═O) OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H (2,2,3,3-tetrafluoropropyl trifluoroacetate), CF₃C(═O)OCH₂CF₃, CF₃C(═O) OCH(CF₃)₂, CH₃C(═O) OCH₂CF₃, ethylpentafluorobutyrate, methyl pentafluoropropionate, ethylpentafluoropropionate, methyl pentafluoroisobutyrate, isopropyltrifluorobutyrate, ethyl trifluoroacetate, tert-butyl trifluoroacetate,n-butyl trifluoroacetate, methyl tetrafluoro-2-(methoxy)propionate,2,2-difluoroethyl acetate, 2,2,3,3-tetrafluoropropyl acetate,2,2,2-trifluoroethyl acetate, 1H,1H-heptafluorobutyl acetate, methyl4,4,4-trifluorobutyrate, ethyl 4,4,4-trifluorobutyrate, ethyl3,3,3-trifluoropropionate, 3,3,3trifluoropropyl3,3,3-trifluoropropionate, ethyl 3-(trifluoromethyl)butyrate, methyl2,3,3,3-tetrafluoropropionate, butyl 2,2-difluoroacetate, methyl2,2,3,3-tetrafluoropropionate, methyl2-(trifluoromethyl)-3,3,3-trifluoropropionate, and methylheptafluorobutyrate.

In order to achieve good miscibility with other solvents and good ratecharacteristics, preferred among these are CF₃CH₂C(═O) OCH₃,HCF₂C(═O)OCH₃, HCF₂C(═O) OC₂H₅, CF₃C(═O) OCH₂C₂F₅, CF₃C(═O) OCH₂CF₂CF₂H,CF₃C(═O) OCH₂CF₃, CF₃C(═O) OCH(CF₃)₂, ethyl pentafluorobutyrate, methylpentafluoropropionate, ethyl pentafluoropropionate, methylpentafluoroisobutyrate, isopropyl trifluorobutyrate, ethyltrifluoroacetate, tert-butyl trifluoroacetate, n-butyl trifluoroacetate,methyl tetrafluoro-2-(methoxy)propionate, 2,2-difluoroethyl acetate,2,2,3,3-tetrafluoropropyl acetate, 2,2,2-trifluoroethyl acetate,1H,1H-heptafluorobutyl acetate, methyl 4,4,4-trifluorobutyrate, ethyl4,4,4-trifluorobutyrate, ethyl 3,3,3-trifluoropropionate,3,3,3-trifluoropropyl 3,3,3-trifluoropropionate, ethyl3-(trifluoromethyl)butyrate, methyl 2,3,3,3-tetrafluoropropionate, butyl2,2-difluoroacetate, methyl 2,2,3,3,-tetrafluoropropionate, methyl2-(trifluoromethyl)-3,3,3-trifluoropropionate, and methylheptafluorobutyrate, more preferred are CF₃CH₂C(═O) OCH₃, HCF₂C(═O)OCH₃, and HCF₂C(═O) OC₂H₅, and particularly preferred are HCF₂C(═O) OCH₃and HCF₂C(═O) OC₂H₅.

One fluorinated acyclic carboxylate may be used alone or two or morethereof may be used in any combination at any ratio.

The fluorinated acyclic carboxylate, when contained, is preferablypresent in an amount of 10 to 90% by volume, more preferably 40 to 85%by volume, still more preferably 50 to 80% by volume, relative to thesolvent.

The solvent preferably contains at least one selected from the groupconsisting of the cyclic carbonate, the acyclic carbonate, and theacyclic carboxylate, and more preferably contains the cyclic carbonateand at least one selected from the group consisting of the acycliccarbonate and the acyclic carboxylate. The cyclic carbonate ispreferably a saturated cyclic carbonate.

An electrolyte solution containing a solvent of such a compositionenables an electrochemical device to have further improvedhigh-temperature storage characteristics and cycle characteristics.

For the solvent containing the cyclic carbonate and at least oneselected from the group consisting of the acyclic carbonate and theacyclic carboxylate, the total amount of the cyclic carbonate and atleast one selected from the group consisting of the acyclic carbonateand the acyclic carboxylate ester is preferably 10 to 100% by volume,more preferably 30 to 100% by volume, still more preferably 50 to 100%by volume.

For the solvent containing the cyclic carbonate and at least oneselected from the group consisting of the acyclic carbonate and theacyclic carboxylate, the cyclic carbonate and at least one selected fromthe group consisting of the acyclic carbonate and the acycliccarboxylate preferably give a volume ratio of 5/95 to 95/5, morepreferably 10/90 or more, still more preferably 15/85 or more,particularly preferably 20/80 or more, while more preferably 90/10 orless, still more preferably 60/40 or less, particularly preferably 50/50or less.

The solvent also preferably contains at least one selected from thegroup consisting of the non-fluorinated saturated cyclic carbonate, thenon-fluorinated acyclic carbonate, and the non-fluorinated acycliccarboxylate, more preferably contains the non-fluorinated saturatedcyclic carbonate and at least one selected from the group consisting ofthe non-fluorinated acyclic carbonate and the non-fluorinated acycliccarboxylate. An electrolyte solution containing a solvent of such acomposition can suitably be used for electrochemical devices used atrelatively low voltage.

For the solvent containing the non-fluorinated saturated cycliccarbonate and at least one selected from the group consisting of thenon-fluorinated acyclic carbonate and the non-fluorinated acycliccarboxylate, the total amount of the non-fluorinated saturated cycliccarbonate and at least one selected from the group consisting of thenon-fluorinated acyclic carbonate and the non-fluorinated acycliccarboxylate ester is preferably 5 to 100% by volume, more preferably 20to 100% by volume, still more preferably 30 to 100% by volume.

For the electrolyte solution containing the non-fluorinated saturatedcyclic carbonate and at least one selected from the group consisting ofthe non-fluorinated acyclic carbonate and the non-fluorinated acycliccarboxylate, the non-fluorinated saturated cyclic carbonate and at leastone selected from the group consisting of the non-fluorinated acycliccarbonate and the non-fluorinated acyclic carboxylate ester preferablygive a volume ratio of 5/95 to 95/5, more preferably 10/90 or more,still more preferably 15/85 or more, particularly preferably 20/80 ormore, while more preferably 90/10 or less, still more preferably 60/40or less, particularly preferably 50/50 or less.

The solvent preferably contains at least one selected from the groupconsisting of the fluorinated saturated cyclic carbonate, thefluorinated acyclic carbonate, and the fluorinated acyclic carboxylate,and more preferably contains the fluorinated saturated cyclic carbonateand at least one selected from the group consisting of the fluorinatedacyclic carbonate and the fluorinated acyclic carboxylate. Anelectrolyte solution containing a solvent of such a composition cansuitably be used for not only electrochemical devices used at relativelyhigh voltage but also electrochemical devices used at relatively lowvoltage.

For the solvent containing the fluorinated saturated cyclic carbonateand at least one selected from the group consisting of the fluorinatedacyclic carbonate and the fluorinated acyclic carboxylate, the totalamount of the fluorinated saturated cyclic carbonate and at least oneselected from the group consisting of the fluorinated acyclic carbonateand the fluorinated acyclic carboxylate ester is preferably 5 to 100% byvolume, more preferably 10 to 100% by volume, still more preferably 30to 100% by volume.

For the solvent containing the fluorinated saturated cyclic carbonateand at least one selected from the group consisting of the fluorinatedacyclic carbonate and the fluorinated acyclic carboxylate, thefluorinated saturated cyclic carbonate and at least one selected fromthe group consisting of the fluorinated acyclic carbonate and thefluorinated acyclic carboxylate ester preferably give a volume ratio of5/95 to 95/5, more preferably 10/90 or more, still more preferably 15/85or more, particularly preferably 20/80 or more, while more preferably90/10 or less, still more preferably 60/40 or less, particularlypreferably 50/50 or less.

The solvent used may be an ionic liquid. The “ionic liquid” means aliquid containing an ion that is a combination of an organic cation andan anion.

Examples of the organic cation include, but are not limited to,imidazolium ions such as dialkyl imidazolium cations and trialkylimidazolium cations; tetraalkyl ammonium ions; alkyl pyridinium ions;dialkyl pyrrolidinium ions; and dialkyl piperidinium ions.

Examples of the anion to be used as a counterion of any of these organiccations include, but are not limited to, a PF₆ anion, a PF₃(C₂F₅)₃anion, a PF₃(CF₃)₃ anion, a BF₄ anion, a BF₂ (CF₃)₂ anion, a BF₃ (CF₃)anion, a bisoxalatoborate anion, a P(C₂O₄)F₂ anion, a Tf(trifluoromethanesulfonyl) anion, Nf (nonafluorobutanesulfonyl) anion, abis(fluorosulfonyl)imide anion, a bis(trifluoromethanesulfonyl)imideanion, a bis(pentafluoroethanesulfonyl)imide anion, a dicyanoamineanion, and halide anions.

The solvent is preferably a non-aqueous solvent and the electrolytesolution of the invention is preferably a non-aqueous electrolytesolution.

The electrolyte solution of the invention may further contain a compound(5) represented by the following formula (5).

The formula (5) is as follows:

wherein

A^(a+) is a metal ion, a hydrogen ion, or an onium ion;

a is an integer of 1 to 3;

b is an integer of 1 to 3;

p is b/a;

n²⁰³ is an integer of 1 to 4;

n²⁰ is an integer of 0 to 8;

n²⁰² is 0 or 1;

Z²⁰¹ is a transition metal or an element in group III, group IV, orgroup V of the Periodic Table;

X²⁰¹ is O, S, a C1-C10 alkylene group, a C1-C10 halogenated alkylenegroup, a C6-C20 arylene group, or a C6-C20 halogenated arylene group,with the alkylene group, the halogenated alkylene group, the arylenegroup, and the halogenated arylene group each optionally containing asubstituent and/or a hetero atom in the structure thereof, and when n²⁰²is 1 and n²⁰³ is 2 to 4, n²⁰³ X²⁰¹s optionally bind to each other;

L²⁰¹ is a halogen atom, a cyano group, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, a C6-C20 halogenated arylgroup, or —Z²⁰³Y²⁰³, with the alkylene group, the halogenated alkylenegroup, the arylene group, and the halogenated arylene group eachoptionally containing a substituent and/or a hetero atom in thestructure thereof, and when n²⁰¹ is 2 to 8, n²⁰¹ L²⁰¹s optionally bindto each other to form a ring;

Y²⁰¹, Y²⁰², and Z²⁰³ are each individually O, S, NY²⁰⁴, a hydrocarbongroup, or a fluorinated hydrocarbon group;

Y²⁰³ and Y²⁰⁴ are each individually H, F, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, or a C6-C20 halogenatedaryl group, with the alkyl group, the halogenated alkyl group, the arylgroup, and the halogenated aryl group each optionally containing asubstituent and/or a hetero atom in the structure thereof, and whenmultiple Y²⁰³s or multiple Y²⁰⁴s are present, they optionally bind toeach other to form a ring.

Examples of A^(a+) include a lithium ion, a sodium ion, a potassium ion,a magnesium ion, a calcium ion, a barium ion, a caesium ion, a silverion, a zinc ion, a copper ion, a cobalt ion, an iron ion, a nickel ion,a manganese ion, a titanium ion, a lead ion, a chromium ion, a vanadiumion, a ruthenium ion, an yttrium ion, lanthanoid ions, actinoid ions, atetrabutyl ammonium ion, a tetraethyl ammonium ion, a tetramethylammonium ion, a triethyl methyl ammonium ion, a triethyl ammonium ion, apyridinium ion, an imidazolium ion, a hydrogen ion, a tetraethylphosphonium ion, a tetramethyl phosphonium ion, a tetraphenylphosphonium ion, a triphenyl sulfonium ion, and a triethyl sulfoniumion.

In applications such as electrochemical devices, A^(a+) is preferably alithium ion, a sodium ion, a magnesium ion, a tetraalkyl ammonium ion,or a hydrogen ion, particularly preferably a lithium ion. The valence aof the cation A^(a+) is an integer of 1 to 3. If the valence a isgreater than 3, the crystal lattice energy is high and the compound (5)has difficulty in dissolving in a solvent. Thus, the valence a is morepreferably 1 when good solubility is needed. The valence b of the anionis also an integer of 1 to 3, particularly preferably 1. The constant pthat represents the ratio between the cation and the anion is naturallydefined by the ratio b/a between the valences thereof.

Next, ligands in the formula (5) are described. The ligands herein meanorganic or inorganic groups binding to Z²⁰¹ in the formula (5).

Z²⁰¹ is preferably Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta,Bi, P, As, Sc, Hf, or Sb, more preferably Al, B, or P.

X²⁰¹ is O, S, a C1-C10 alkylene group, a C1-C10 halogenated alkylenegroup, a C6-C20 arylene group, or a C6-C20 halogenated arylene group.These alkylene groups and arylene groups each may have a substituentand/or a hetero atom in the structure. Specifically, instead of ahydrogen atom in the alkylene group or the arylene group, the structuremay have a halogen atom, a linear or cyclic alkyl group, an aryl group,an alkenyl group, an alkoxy group, an aryloxy group, a sulfonyl group,an amino group, a cyano group, a carbonyl group, an acyl group, an amidegroup, or a hydroxy group as a substituent; or, instead of a carbon atomin the alkylene or the arylene, the structure may have nitrogen, sulfur,or oxygen introduced therein. When n²⁰² is 1 and n²⁰³ is 2 to 4, n²⁰³X²⁰¹s may bind to each other. One such example is a ligand such asethylenediaminetetraacetic acid.

L²⁰¹ is a halogen atom, a cyano group, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, a C6-C20 halogenated arylgroup, or —Z²⁰³Y²⁰³ (Z²⁰³ and Y²⁰³ will be described later). Similar toX²⁰¹, the alkyl groups and the aryl groups each may have a substituentand/or a hetero atom in the structure, and when n²⁰¹ is 2 to 8, n²⁰¹L²⁰¹s optionally bind to each other to form a ring. L²⁰¹ is preferably afluorine atom or a cyano group. This is because a fluorine atom canimprove the solubility and the degree of dissociation of a salt of ananion compound, thereby improving the ion conductivity. This is alsobecause a fluorine atom can improve the oxidation resistance, reducingoccurrence of side reactions.

Y²⁰¹, Y²⁰², and Z²⁰³ are each individually O, S, NY²⁰⁴, a hydrocarbongroup, or a fluorinated hydrocarbon group. Y²⁰¹ and Y²⁰² are eachpreferably O, S, or NY²⁰⁴, more preferably O. The compound (5)characteristically has a bond between Y²⁰¹ and Z²⁰¹ and a bond betweenY²⁰² and Z²⁰¹ in the same ligand. Such a ligand forms a chelatestructure with Z²⁰¹. This chelate has an effect of improving the heatresistance, the chemical stability, and the hydrolysis resistance ofthis compound. The constant n²⁰² of the ligand is 0 or 1. In particular,n²² is preferably 0 because the chelate ring becomes a five-memberedring, leading to the most strongly exerted chelate effect and improvedstability.

The term “fluorinated hydrocarbon group” herein means a group obtainableby replacing at least one hydrogen atom of a hydrocarbon group by afluorine atom.

Y²⁰³ and Y²⁰⁴ are each individually H, F, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, or a C6-C20 halogenatedaryl group. These alkyl groups and aryl groups each may contain asubstituent or a hetero atom in the structure. When multiple Y²⁰³s ormultiple Y²⁰⁴s are present, they optionally bind to each other to form aring.

The constant n²⁰³ relating to the number of the aforementioned ligandsis an integer of 1 to 4, preferably 1 or 2, more preferably 2. Theconstant n²⁰¹ relating to the number of the aforementioned ligands is aninteger of 0 to 8, preferably an integer of 0 to 4, more preferably 0,2, or 4. In addition, when n²⁰³ is 1, n²⁰¹ is preferably 2; and whenn²⁰³ is 2, n²⁰¹ is preferably 0.

In the formula (5), the alkyl group, the halogenated alkyl group, thearyl group, and the halogenated aryl group include those having anyother functional groups such as branches, hydroxy groups, and etherbonds.

The compound (5) is preferably a compound represented by the followingformula:

(wherein A^(a+), a, b, p, n²⁰¹, Z²⁰¹, and L²⁰¹ are defined as describedabove), or a compound represented by the following formula:

(wherein A^(a+), a, b, p, n²⁰¹, Z²⁰¹, and L²⁰¹ are defined as describedabove).

The compound (5) may be a lithium oxalatoborate salt. Examples thereofinclude lithium bis(oxalato)borate (LiBOB) represented by the followingformula:

and lithium difluorooxalatoborate (LiDFOB) represented by the followingformula:

Examples of the compound (5) also include

lithium difluorooxalatophosphanite (LiDFOP) represented by the followingformula:

lithium tetrafluorooxalatophosphanite (LITFOP) represented by thefollowing formula:

and

lithium bis(oxalato)difluorophosphanite represented by the followingformula:

In addition, specific examples of dicarboxylic acid complex saltscontaining boron as a comlex center element include lithiumbis(malonato)borate, lithium difluoro(malonato)borate, lithiumbis(methylmalonato)borate, lithium difluoro(methylmalonato)borate,lithium bis(dimethylmalonato)borate, and lithiumdifluoro(dimethylmalonato)borate.

Specific examples of dicarboxylic acid complex salts containingphosphorus as a complex center element include lithiumtris(oxalato)phosphate, lithium tris(malonato)phosphate, lithiumdifluorobis(malonato)phosphate, lithium tetrafluoro(malonato)phosphate,lithium tris(methylmalonato)phosphate, lithiumdifluorobis(methylmalonato)phosphate, lithiumtetrafluoro(methylmalonato)phosphate, lithiumtris(dimethylmalonato)phosphate, lithiumdifluorobis(dimethylmalonato)phosphate, and lithiumtetrafluoro(dimethylmalonato)phosphate.

Specific examples of dicarboxylic acid complex salts containing aluminumas a complex center element include LiAl(C₂O₄)₂ and LiAlF₂(C₂O₄).

In order to enable easy availability and contribute to formation of astable film-shaped structure, more preferred among these are lithiumbis(oxalato)borate, lithium difluoro(oxalato)borate, lithiumtris(oxalato)phosphate, lithium difluorobis(oxalato)phosphate, andlithium tetrafluoro(oxalato)phosphate.

In order to give much better cycle characteristics, the compound (5) ispreferably in an amount of 0.001% by mass or more, more preferably 0.01%by mass or more, while preferably 10% by mass or less, more preferably3% by mass or less, relative to the solvent.

The electrolyte solution of the invention preferably further contains anelectrolyte salt other than the compound (5). Examples of theelectrolyte salt used include lithium salts, ammonium salts, and metalsalts, as well as any of those to be used for electrolyte solutions suchas liquid salts (ionic liquids), inorganic polymer salts, and organicpolymer salts.

The electrolyte salt of the electrolyte solution for a lithium ionsecondary battery is preferably a lithium salt.

Any lithium salt may be used. Specific examples thereof include thefollowing:

inorganic lithium salts such as LiPF₆, LiBF₄, LiClO₄, LiAlF₄, LiSbF₆,LiTaF₆, LiWF₇, LiAsF₆, LiAlCl₄, LiI, LiBr, LiCl, LiB₁₀Cl₁₀, Li₂SiF₆,Li₂PFO₃, and LiPO₂F₂;

lithium tungstates such as LiWOF₅;

lithium carboxylates such as HCO₂Li, CH₃CO₂Li, CH₂FCO₂Li, CHF₂CO₂Li,CF₃CO₂Li, CF₃CH₂CO₂Li, CF₃CF₂CO₂Li, CF₃CF₂CF₂CO₂Li, andCF₃CF₂CF₂CF₂CO₂Li;

lithium salts containing an S═O group such as FSO₃Li, CH₃SO₃Li,CH₂FSO₃Li, CHF₂SO₃Li, CF₃SO₃Li, CF₃CF₂SO₃Li, CF₃CF₂CF₂SO₃Li,CF₃CF₂CF₂CF₂SO₃Li, lithium methylsulfate, lithium ethylsulfate(C₂H₅OSO₃Li), and lithium 2,2,2-trifluoroethylsulfate;

lithium imide salts such as LiN(FCO)₂, LiN(FCO) (FSO₂), LiN(FSO₂)₂,LiN(FSO₂) (CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithiumbis-perfluoroethanesulfonyl imide, lithium cyclic1,2-perfluoroethanedisulfonyl imide, lithium cyclic1,3-perfluoropropanedisulfonyl imide, lithium cyclic1,2-ethanedisulfonyl imide, lithium cyclic 1,3-propanedisulfonyl imide,lithium cyclic 1,4-perfluorobutanedisulfonyl imide, LiN(CF₃SO₂) (FSO₂),LiN(CF₃SO₂) (C₃F₇SO₂), LiN(CF₃SO₂) (C₄F₉SO₂), and LiN(POF₂)₂;

lithium methide salts such as LiC(FSO₂)₃, LiC(CF₃SO₂)₃, andLiC(C₂F₅SO₂)₃; and

fluorine-containing organic lithium salts such as salts represented bythe formula: LiPF_(a)(C_(n)F_(2n+1))_(6-a) (wherein a is an integer of 0to 5; and n is an integer of 1 to 6) such as LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃,LiPF₃(iso-C₃F₇)₃, LiPF₅(iso-C₃F₇), LiPF₄(CF₃) 2, and LiPF₄(C₂F₅)₂, aswell as LiPF₄ (CF₃SO₂) 2, LiPF₄ (C₂F₅SO₂) 2, LiBF₃CF₃, LiBF₃C₂F₅,LiBF₃C₃F₇, LiBF₂ (CF₃) 2, LiBF₂ (C₂F₅) 2, LiBF₂ (CF₃SO₂) 2, and LiBF₂(C₂F₅SO₂) 2, and LiSCN, LiB(CN)₄, LiB(C₆H₅)₄, Li₂(C₂O₄), LiP(C₂O₄)₃, andLi₂B₁₂F_(b)H_(12-b) (wherein b is an integer of 0 to 3).

In order to achieve an effect of improving properties such as outputcharacteristics, high-rate charge and discharge characteristics,high-temperature storage characteristics, and cycle characteristics,particularly preferred among these are LiPF₆, LiBF₄, LiSbF₆, LiTaF₆,LiPO₂F₂, FSO₃Li, CF₃SO₃Li, LiN(FSO₂)₂, LiN(FSO₂) (CF₃SO₂), LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, lithium cyclic 1,2-perfluoroethanedisulfonyl imide,lithium cyclic 1,3-perfluoropropanedisulfonyl imide, LiC(FSO₂)₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂) 3, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃ (CF₃) 3, LiPF₃(C₂F₅) 3, and the like. Most preferred is at least one lithium saltselected from the group consisting of LiPF₆, LiN(FSO₂)₂, and LiBF₄.

One of these electrolyte salts may be used alone, or two or more thereofmay be used in combination. Preferred examples of the combination forusing two or more electrolyte salts include a combination of LiPF₆ andLiBF₄, a combination of LiPF₆ and FSO₃Li, and a combination of LiPF₆ andLiPO₂F₂. These combinations have an effect of improving loadcharacteristics and cycle characteristics.

In the case of combination use, the amount of LiBF₄, FSO₃Li, or LiPO₂F₂relative to 100% by mass of the entire electrolyte solution is notlimited and may be appropriately set as long as the effects of theinvention are not significantly impaired. The amount thereof is usually0.01% by mass or more, preferably 0.1% by mass or more, while 30% bymass or less, preferably 20% by mass, relative to the electrolytesolution of the invention.

In another example, an inorganic lithium salt and an organic lithiumsalt are used in combination. Such a combination has an effect ofreducing deterioration due to high-temperature storage. The organiclithium salt is preferably CF₃SO₃Li, LiN(FSO₂)₂, LiN(FSO₂) (CF₃SO₂),LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium cyclic1,2-perfluoroethanedisulfonyl imide, lithium cyclic1,3-perfluoropropanedisulfonyl imide, LiC(FSO₂)₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂) 3, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃ (CF₃) 3, LiPF₃ (C₂F₅) 3, orthe like. In this case, the proportion of the organic lithium salt ispreferably 0.1% by mass or more, particularly preferably 0.5% by mass ormore, while preferably 30% by mass or less, particularly preferably 20%by mass or less, of 100% by mass of the whole electrolyte solution.

The electrolyte salt in the electrolyte solution may have anyconcentration that does not impair the effects of the invention. Inorder to make the electric conductivity of the electrolyte solutionwithin a favorable range and to ensure good battery performance, thelithium in the electrolyte solution preferably has a total moleconcentration of 0.3 mol/L or higher, more preferably 0.4 mol/L orhigher, still more preferably 0.5 mol/L or higher, while preferably 3mol/L or lower, more preferably 2.5 mol/L or lower, still morepreferably 2.0 mol/L or lower.

Too low a total mole concentration of lithium may cause insufficientelectric conductivity of the electrolyte solution, while too high aconcentration may cause an increase in viscosity and then reduction inelectric conductivity, impairing the battery performance.

The electrolyte salt in the electrolyte solution for an electric doublelayer capacitor is preferably an ammonium salt.

Examples of the ammonium salt include the following salts (IIa) to(IIe).

(IIa) Tetraalkyl Quaternary Ammonium Salts

Preferred examples thereof include tetraalkyl quaternary ammonium saltsrepresented by the following formula (IIa):

(wherein R^(1a), R^(2a), R^(3a), and R^(4a) are the same as or differentfrom each other, and are each a C1-C6 alkyl group optionally containingan ether bond; and X⁻ is an anion) In order to improve the oxidationresistance, any or all of the hydrogen atoms in the ammonium salt arealso preferably replaced by a fluorine atom and/or a C1-C4fluorine-containing alkyl group.

Preferred specific examples thereof include

tetraalkyl quaternary ammonium salts represented by the followingformula (IIa-1):

[Chem. 67]

(R^(1a))_(x)(R^(2a))_(y)N^(⊕)X^(⊕)  (IIa-1)

(wherein R^(1a), R^(2a), and X⁻ are defined as described above; x and yare the same as or different from each other, and are each an integer of0 to 4 with x+y=4), and

alkyl ether group-containing trialkyl ammonium salts represented by thefollowing formula (IIa-2):

(wherein R^(5a) is a C1-C6 alkyl group; R^(6a) is a C1-C6 divalenthydrocarbon group; R^(7a) is a C1-C4 alkyl group; z is 1 or 2; and X⁻ isan anion). Introduction of an alkyl ether group enables reduction inviscosity.

The anion X⁻ may be either an inorganic anion or an organic anion.Examples of the inorganic anion include AlCl₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻,TaF₆ ⁻, I⁻, and SbF₆ ⁻. Examples of the organic anion include abisoxalatoborate anion, a difluorooxalatoborate anion, atetrafluorooxalatophosphate anion, a difluorobisoxalatophosphate anion,CF₃COO⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻ and (C₂F₅SO₂) 2N⁻.

In order to achieve good oxidation resistance and ionic dissociation,preferred are BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, and SbF₆ ⁻.

Preferred specific examples of the tetraalkyl quaternary ammonium saltsto be used include Et₄NBF₄, Et₄NClO₄, Et₄NPF₆, Et₄NAsF₆, Et₄NSbF₆,Et₄NCF₃SO₃, Et₄N(CF₃SO₂)₂N, Et₄NC₄F₉SO₃, Et₃MeNBF₄, Et₃MeNClO₄,Et₃MeNPF₆, Et₃MeNAsF₆, Et₃MeNSbF₆, Et₃MeNCF₃SO₃, Et₃MeN(CF₃SO₂)₂N, andEt₃MeNC₄F₉SO₃. In particular, Et₄NBF₄, Et₄NPF₆, Et₄NSbF₆, Et₄NAsF₆,Et₃MeNBF₄, and an N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium saltmay be mentioned as examples.

(IIb) Spirocyclic Bipyrrolidinium Salts

Preferred examples thereof include

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-1):

(wherein R^(8a) and R^(9a) are the same as or different from each other,and are each a C1-C4 alkyl group; X⁻ is an anion; n1 is an integer of 0to 5; and n2 is an integer of 0 to 5),

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-2):

(wherein R^(10a) and R^(11a) are the same as or different from eachother, and are each a C1-C4 alkyl group; X⁻ is an anion; n3 is aninteger of 0 to 5; and n4 is an integer of 0 to 5), and

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-3):

(wherein R^(12a) and R^(13a) are the same as or different from eachother, and are each a C1-C4 alkyl group; X⁻ is an anion; n5 is aninteger of 0 to 5; and n6 is an integer of 0 to 5). In order to improvethe oxidation resistance, any or all of the hydrogen atoms in thespirocyclic bipyrrolidinium salt are also preferably replaced by afluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa). In order to achieve good dissociation anda low internal resistance under high voltage, preferred among these isBF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, or (C₂F₅SO₂)₂N.

For example, those represented by the following formulae:

may be mentioned as preferred specific examples of the spirocyclicbipyrrolidinium salts.

These spirocyclic bipyrrolidinium salts are excellent in solubility in asolvent, oxidation resistance, and ion conductivity.

(IIc) Imidazolium Salts

Preferred examples thereof include imidazolium salts represented by thefollowing formula (IIc):

wherein R^(14a) and R^(15a) are the same as or different from eachother, and are each a C1-C6 alkyl group; and X⁻ is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the imidazolium salt are also preferably replaced by a fluorineatom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, one represented by the following formula:

may be mentioned as a preferred specific example thereof.

This imidazolium salt is excellent in that it has low viscosity and goodsolubility.

(IId)N-Alkylpyridinium Salts

Preferred examples thereof include N-alkylpyridinium salts representedby the following formula (IId):

wherein R^(16a) is a C1-C6 alkyl group; and X⁻ is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the N-alkylpyridinium salt are also preferably replaced by afluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulae:

may be mentioned as preferred specific examples thereof.

These N-alkylpyridinium salts are excellent in that they have lowviscosity and good solubility.

(IIe) N,N-Dialkylpyrrolidinium Salts

Preferred examples thereof include N,N-dialkylpyrrolidinium saltsrepresented by the following formula (IIe):

wherein R^(17a) and R^(18a) are the same as or different from eachother, and are each a C1-C6 alkyl group; and X⁻ is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the N,N-dialkylpyrrolidinium salt are also preferably replacedby a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulae:

may be mentioned as preferred specific examples thereof.

These N,N-dialkylpyrrolidinium salts are excellent in that they have lowviscosity and good solubility.

Preferred among these ammonium salts are those represented by theformula (IIa), (IIb), or (IIc) because they can have good solubility,oxidation resistance, and ion conductivity. More preferred are thoserepresented by the following formulae:

wherein Me is a methyl group; Et is an ethyl group; and X⁻, x, and y aredefined as in the formula (IIa-1).

A lithium salt may be used as an electrolyte salt for an electric doublelayer capacitor. Preferred examples thereof include LiPF₆, LiBF₄,LiN(FSO₂)₂, LiAsF₆, LiSbF₆, and LiN(SO₂C₂H₅)₂.

In order to further increase the capacity, a magnesium salt may be used.Preferred examples of the magnesium salt include Mg(ClO₄)₂ andMg(OOC₂H₅)₂.

The ammonium salt serving as an electrolyte salt is preferably used at aconcentration of 0.7 mol/L or higher. The ammonium salt at aconcentration lower than 0.7 mol/L may cause not only poorlow-temperature characteristics but also high initial internalresistance. The concentration of the electrolyte salt is more preferably0.9 mol/L or higher.

In order to give good low-temperature characteristics, the upper limitof the concentration is preferably 2.0 mol/L or lower, more preferably1.5 mol/L or lower.

When the ammonium salt is triethyl methyl ammonium tetrafluoroborate(TEMABF₄), the concentration is preferably 0.7 to 1.5 mol/L to giveexcellent low-temperature characteristics.

When the ammonium salt is spirobipyrrolidinium tetrafluoroborate(SBPBF₄) the concentration is preferably 0.7 to 2.0 mol/L.

The electrolyte solution of the invention preferably further contains acompound (2) represented by the following formula (2):

(wherein X²¹ is a group containing at least H or C; n²¹ is an integer of1 to 3; Y²¹ and Z²¹ are the same as or different from each other, andare each a group containing at least H, C, O, or F; n²² is 0 or 1; andY²¹ and Z²¹ optionally bind to each other to form a ring). Theelectrolyte solution containing the compound (2) can cause much lessreduction in capacity retention and can cause a much less increase inamount of gas generated even when stored at high temperature.

When n²¹ is 2 or 3, the two or three X²¹s may be the same as ordifferent from each other.

When multiple Y²¹s and multiple Z²¹s are present, the multiple Y²¹s maybe the same as or different from each other and the multiple Z²¹s may bethe same as or different from each other.

X²¹ is preferably a group represented by —CY²¹Z²¹— (wherein Y²¹ and Z²¹are defined as described above) or a group represented by —CY²¹═CZ²¹—(wherein Y²¹ and Z²¹ are defined as described above).

Y²¹ preferably includes at least one selected from the group consistingof H—, F—, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CF₃—, CF₃CF₂—, CH₂FCH₂—, andCF₃CF₂CF₂—.

Z²¹ preferably includes at least one selected from the group consistingof H—, F—, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CF₃—, CF₃CF₂—, CH₂FCH₂—, andCF₃CF₂CF₂—.

Alternatively, Y²¹ and Z²¹ may bind to each other to form a carbon ringor heterocycle that may contain an unsaturated bond and may havearomaticity. The ring preferably has a carbon number of 3 to 20.

Next, specific examples of the compound (2) are described. In thefollowing examples, the term “analog” means an acid anhydride obtainableby replacing part of the structure of an acid anhydride mentioned as anexample by another structure within the scope of the invention. Examplesthereof include dimers, trimers, and tetramers each composed of aplurality of acid anhydrides, structural isomers such as those having asubstituent that has the same carbon number but also has a branch, andthose having a different site at which a substituent binds to the acidanhydride.

Specific examples of an acid anhydride having a 5-membered cyclicstructure include succinic anhydride, methylsuccinic anhydride(4-methylsuccinic anhydride), dimethylsuccinic anhydride (e.g.,4,4-dimethylsuccinic anhydride, 4,5-dimethylsuccinic anhydride),4,4,5-trimethylsuccinic anhydride, 4,4,5,5-tetramethylsuccinicanhydride, 4-vinylsuccinic anhydride, 4,5-divinylsuccinic anhydride,phenylsuccinic anhydride (4-phenylsuccinic anhydride),4,5-diphenylsuccinic anhydride, 4,4-diphenylsuccinic anhydride,citraconic anhydride, maleic anhydride, methylmaleic anhydride(4-methylmaleic anhydride), 4,5-dimethylmaleic anhydride, phenylmaleicanhydride (4-phenylmaleic anhydride), 4,5-diphenylmaleic anhydride,itaconic anhydride, 5-methylitaconic anhydride, 5,5-dimethylitaconicanhydride, phthalic anhydride, and 3,4,5,6-tetrahydrophthalic anhydride,and analogs thereof.

Specific examples of an acid anhydride having a 6-membered cyclicstructure include cyclohexanedicarboxylic anhydride (e.g.,cyclohexane-1,2-dicarboxylic anhydride), 4-cyclohexene-1,2-dicarboxylicanhydride, glutaric anhydride, glutaconic anhydride, and2-phenylglutaric anhydride, and analogs thereof.

Specific examples of an acid anhydride having a different cyclicstructure include 5-norbornene-2,3-dicarboxylic anhydride,cyclopentanetetracarboxylic dianhydride, pyromellitic anhydride, anddiglycolic anhydride, and analogs thereof.

Specific examples of an acid anhydride having a cyclic structure andsubstituted with a halogen atom include monofluorosuccinic anhydride(e.g., 4-fluorosuccinic anhydride), 4,4-difluorosuccinic anhydride,4,5-difluorosuccinic anhydride, 4,4,5-trifluorosuccinic anhydride,trifluoromethylsuccinic anhydride, tetrafluorosuccinic anhydride(4,4,5,5-tetrafluorosuccinic anhydride), 4-fluoromaleic anhydride,4,5-difluoromaleic anhydride, trifluoromethylmaleic anhydride,5-fluoroitaconic anhydride, and 5,5-difluoroitaconic anhydride, andanalogs thereof.

Preferred among these as the compound (2) are glutaric anhydride,citraconic anhydride, glutaconic anhydride, itaconic anhydride,diglycolic anhydride, cyclohexanedicarboxylic anhydride,cyclopentanetetracarboxylic dianhydride, 4-cyclohexene-1,2-dicarboxylicanhydride, 3,4,5,6-tetrahydrophthalic anhydride,5-norbornene-2,3-dicarboxylic anhydride, phenylsuccinic anhydride,2-phenylglutaric anhydride, maleic anhydride, methylmaleic anhydride,trifluoromethylmaleic anhydride, phenylmaleic anhydride, succinicanhydride, methylsuccinic anhydride, dimethylsuccinic anhydride,trifluoromethylsuccinic anhydride, monofluorosuccinic anhydride, andtetrafluorosuccinic anhydride. More preferred are maleic anhydride,methylmaleic anhydride, trifluoromethylmaleic anhydride, succinicanhydride, methylsuccinic anhydride, trifluoromethylsuccinic anhydride,and tetrafluorosuccinic anhydride, and still more preferred are maleicanhydride and succinic anhydride.

The compound (2) preferably includes at least one selected from thegroup consisting of: a compound (3) represented by the following formula(3):

(wherein X³¹ to X³⁴ are the same as or different from each other, andare each a group containing at least H, C, O, or F); and a compound (4)represented by the following formula (4):

(wherein X⁴¹ and X⁴² are the same as or different from each other, andare each a group containing at least H, C, O, or F).

X³¹ to X³⁴ are the same as or different from each other, and preferablyinclude at least one selected from the group consisting of an alkylgroup, a fluorinated alkyl group, an alkenyl group, and a fluorinatedalkenyl group. X³¹ to X³⁴ each preferably have a carbon number of 1 to10, more preferably 1 to 3.

X³¹ to X³⁴ are the same as or different from each other, and morepreferably include at least one selected from the group consisting ofH—, F—, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CF₃—, CF₃CF₂—, CH₂FCH₂—, andCF₃CF₂CF₂—.

X⁴¹ and X⁴² are the same as or different from each other, and preferablyinclude at least one selected from the group consisting of an alkylgroup, a fluorinated alkyl group, an alkenyl group, and a fluorinatedalkenyl group. X⁴¹ and X⁴² each preferably have a carbon number of 1 to10, more preferably 1 to 3.

X⁴¹ and X⁴² are the same as or different from each other, and morepreferably include at least one selected from the group consisting ofH—, F—, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CF₃—, CF₃CF₂—, CH₂FCH₂—, andCF₃CF₂CF₂—.

The compound (3) is preferably any of the following compounds.

The compound (4) is preferably any of the following compounds.

In order to cause much less reduction in capacity retention and a muchless increase in amount of gas generated even when stored at hightemperature, the electrolyte solution preferably contains 0.0001 to 15%by mass of the compound (2) relative to the electrolyte solution. Theamount of the compound (2) is more preferably 0.01 to 10% by mass, stillmore preferably 0.1 to 3% by mass, particularly preferably 0.1 to 1.0%by mass.

In order to cause much less reduction in capacity retention and a muchless increase in amount of gas generated even when stored at hightemperature, the electrolyte solution, when containing both thecompounds (3) and (4), preferably contains 0.08 to 2.50% by mass of thecompound (3) and 0.02 to 1.50% by mass of the compound (4), morepreferably 0.80 to 2.50% by mass of the compound (3) and 0.08 to 1.50%by mass of the compound (4), relative to the electrolyte solution.

The electrolyte solution of the invention may contain at least oneselected from the group consisting of nitrile compounds represented bythe following formulae (1a), (1b), and (1c):

(wherein R^(a) and R^(b) are each individually a hydrogen atom, a cyanogroup (CN), a halogen atom, an alkyl group, or a group obtainable byreplacing at least one hydrogen atom of an alkyl group by a halogenatom; and n is an integer of 1

(wherein R^(c) is a hydrogen atom, a halogen atom, an alkyl group, agroup obtainable by replacing at least one hydrogen atom of an alkylgroup by a halogen atom, or a group represented by NC—R^(c1)—X^(c1)—(wherein R^(c1) is an alkylene group, X^(c1) is an oxygen atom or asulfur atom); R^(d) and R^(e) are each individually a hydrogen atom, ahalogen atom, an alkyl group, or a group obtainable by replacing atleast one hydrogen atom of an alkyl group by a halogen atom; and m is aninteger of 1 to 10);

(wherein R^(f), R^(g), R^(h), and R^(i) are each individually a groupcontaining a cyano group (CN), a hydrogen atom (H), a halogen atom, analkyl group, or a group obtainable by replacing at least one hydrogenatom of an alkyl group by a halogen atom; at least one selected fromR^(f), R^(g), R^(h), and R^(i) is a group containing a cyano group; and1 is an integer of 1 to 3).

This can improve the high-temperature storage characteristics of anelectrochemical device. One nitrile compound may be used alone, or twoor more thereof may be used in any combination at any ratio.

In the formula (1a), R^(a) and R^(b) are each individually a hydrogenatom, a cyano group (CN), a halogen atom, an alkyl group, or a groupobtainable by replacing at least one hydrogen atom of an alkyl group bya halogen atom.

Examples of the halogen atom include a fluorine atom, a chlorine atom, abromine atom, and an iodine atom. Preferred among these is a fluorineatom.

The alkyl group is preferably a C1-C5 alkyl group. Specific examples ofthe alkyl group include a methyl group, an ethyl group, a propyl group,an isopropyl group, a butyl group, an isobutyl group, and a tert-butylgroup.

An example of the group obtainable by replacing at least one hydrogenatom of an alkyl group by a halogen atom is a group obtainable byreplacing at least one hydrogen atom of the aforementioned alkyl groupby the aforementioned halogen atom.

When R^(a) and R^(b) are alkyl groups or groups each obtainable byreplacing at least one hydrogen atom of an alkyl group by a halogenatom, R^(a) and R^(b) may bind to each other to form a cyclic structure(e.g., a cyclohexane ring).

R^(a) and R^(b) are each preferably a hydrogen atom or an alkyl group.

In the formula (1a), n is an integer of 1 to 10. When n is 2 or greater,all of n R^(a)s may be the same as each other, or at least part of themmay be different from the others. The same applies to R^(b). In theformula, n is preferably an integer of 1 to 7, more preferably aninteger of 2 to 5.

Preferred as the nitrile compound represented by the formula (1a) aredinitriles and tricarbonitriles.

Specific examples of the dinitriles include malononitrile,succinonitrile, glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, azelanitrile, sebaconitrile, undecanedinitrile,dodecanedinitrile, methylmalononitrile, ethylmalononitrile,isopropylmalononitrile, tert-butylmalononitrile, methylsuccinonitrile,2,2-dimethylsuccinonitrile, 2,3-dimethylsuccinonitrile,2,3,3-trimethylsuccinonitrile, 2,2,3,3-tetramethylsuccinonitrile,2,3-diethyl-2,3-dimethylsuccinonitrile,2,2-diethyl-3,3-dimethylsuccinonitrile, bicyclohexyl-1,1-dicarbonitrile,bicyclohexyl-2,2-dicarbonitrile, bicyclohexyl-3,3-dicarbonitrile,2,5-dimethyl-2,5-hexanedicarbonitrile,2,3-diisobutyl-2,3-dimethylsuccinonitrile,2,2-diisobutyl-3,3-dimethylsuccinonitrile, 2-methylglutaronitrile,2,3-dimethylglutaronitrile, 2,4-dimethylglutaronitrile,2,2,3,3-tetramethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile,2,2,3,4-tetramethylglutaronitrile, 2,3,3,4-tetramethylglutaronitrile,1,4-dicyanopentane, 2,6-dicyanoheptane, 2,7-dicyanooctane,2,8-dicyanononane, 1,6-dicyanodecane, 1,2-dicyanobenzene,1,3-dicyanobenzene, 1,4-dicyanobenzene,3,3′-(ethylenedioxy)dipropionitrile,3,3′-(ethylenedithio)dipropionitrile,3,9-bis(2-cyanoethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane,butanenitrile, and phthalonitrile. Particularly preferred among theseare succinonitrile, glutaronitrile, and adiponitrile.

Specific examples of the tricarbonitriles includepentanetricarbonitrile, propanetricarbonitrile,1,3,5-hexanetricarbonitrile, 1,3,6-hexanetricarbonitrile,heptanetricarbonitrile, 1,2,3-propanetricarbonitrile,1,3,5-pentanetricarbonitrile, cyclohexanetricarbonitrile,triscyanoethylamine, triscyanoethoxypropane, tricyanoethylene, andtris(2-cyanoethyl)amine.

Particularly preferred are 1,3,6-hexanetricarbonitrile andcyclohexanetricarbonitrile, most preferred iscyclohexanetricarbonitrile.

In the formula (1b), R^(c) is a hydrogen atom, a halogen atom, an alkylgroup, a group obtainable by replacing at least one hydrogen atom of analkyl group by a halogen atom, or a group represented byNC—R^(c1)—X^(c1)— (wherein R^(c1) is an alkylene group; and X^(c1) is anoxygen atom or a sulfur atom); R^(d) and R^(e) are each individually ahydrogen atom, a halogen atom, an alkyl group, or a group obtainable byreplacing at least one hydrogen atom of an alkyl group by a halogenatom.

Examples of the halogen atom, the alkyl group, and the group obtainableby replacing at least one hydrogen atom of an alkyl group by a halogenatom include those mentioned as examples thereof for the formula (1a).

R^(c1) in NC—R^(c1)—X^(c1)— is an alkylene group. The alkylene group ispreferably a C1-C3 alkylene group.

R^(c), R^(d), and R^(e) are each preferably individually a hydrogenatom, a halogen atom, an alkyl group, or a group obtainable by replacingat least one hydrogen atom of an alkyl group by a halogen atom.

At least one selected from R^(c), R^(d), and R^(e) is preferably ahalogen atom or a group obtainable by replacing at least one hydrogenatom of an alkyl group by a halogen atom, more preferably a fluorineatom or a group obtainable by replacing at least one hydrogen atom of analkyl group by a fluorine atom.

When R^(d) and R^(e) are each an alkyl group or a group obtainable byreplacing at least one hydrogen atom of an alkyl group by a halogenatom, R^(d) and R^(e) may bind to each other to form a cyclic structure(e.g., a cyclohexane ring).

In the formula (1b), m is an integer of 1 to 10. When m is 2 or greater,all of m R^(d)s may be the same as each other, or at least part of themmay be different from the others. The same applies to R^(e). In theformula, m is preferably an integer of 2 to 7, more preferably aninteger of 2 to 5.

Examples of the nitrile compound represented by the formula (1b) includeacetonitrile, propionitrile, butyronitrile, isobutyronitrile,valeronitrile, isovaleronitrile, lauronitrile, 3-methoxypropionitrile,2-methylbutyronitrile, trimethylacetonitrile, hexanenitrile,cyclopentanecarbonitrile, cyclohexanecarbonitrile, fluoroacetonitrile,difluoroacetonitrile, trifluoroacetonitrile, 2-fluoropropionitrile,3-fluoropropionitrile, 2,2-difluoropropionitrile,2,3-difluoropropionitrile, 3,3-difluoropropionitrile,2,2,3-trifluoropropionitrile, 3,3,3-trifluoropropionitrile,3,3′-oxydipropionitrile, 3,3′-thiodipropionitrile,pentafluoropropionitrile, methoxyacetonitrile, and benzonitrile.Particularly preferred among these is 3,3,3-trifluoropropionitrile.

In the formula (1c), R^(f), R^(g), R^(h), and R^(i) are eachindividually a group containing a cyano group (CN), a hydrogen atom, ahalogen atom, an alkyl group, or a group obtainable by replacing atleast one hydrogen atom of an alkyl group by a halogen atom.

Examples of the halogen atom, the alkyl group, and the group obtainableby replacing at least one hydrogen atom of an alkyl group by a halogenatom include those mentioned as examples thereof for the formula (1a).

Examples of the group containing a cyano group include a cyano group anda group obtainable by replacing at least one hydrogen atom of an alkylgroup by a cyano group. Examples of the alkyl group in this case includethose mentioned as examples for the formula (1a).

At least one selected from R^(f), R^(g), R^(h), and R^(i) is a groupcontaining a cyano group. Preferably, at least two selected from R^(f),R^(g), R^(h), and R^(i) are each a group containing a cyano group. Morepreferably, R^(h) and R^(i) are each a group containing a cyano group.When R^(h) and R^(i) are each a group containing a cyano group, R^(f)and R^(g) are preferably hydrogen atoms.

In the formula (1c), 1 is an integer of 1 to 3. When 1 is 2 or greater,all of 1 R^(f)s may be the same as each other, or at least part of themmay be different from the others. The same applies to R^(g). In theformula, 1 is preferably an integer of 1 or 2.

Examples of the nitrile compound represented by the formula (1c) include3-hexenedinitrile, mucononitrile, maleonitrile, fumaronitrile,acrylonitrile, methacrylonitrile, crotononitrile,3-methylcrotononitrile, 2-methyl-2-butenenitrile, 2-pentenenitrile,2-methyl-2-pentenenitrile, 3-methyl-2-pentenenitrile, and2-hexenenitrile. Preferred are 3-hexenedinitrile and mucononitrile,particularly preferred is 3-hexenedinitrile.

The nitrile compounds are preferably present in an amount of 0.2 to 7%by mass relative to the electrolyte solution. This can further improvethe high-temperature storage characteristics and safety of anelectrochemical device at high voltage. The lower limit of the totalamount of the nitrile compounds is more preferably 0.3% by mass, stillmore preferably 0.5% by mass. The upper limit thereof is more preferably5% by mass, still more preferably 2% by mass, particularly preferably0.5% by mass.

The electrolyte solution of the invention may contain a compoundcontaining an isocyanate group (hereinafter, also abbreviated as“isocyanate”). The isocyanate used may be any isocyanate. Examples ofthe isocyanate include monoisocyanates, diisocyanates, andtriisocyanates.

Specific examples of the monoisocyanate include isocyanatomethane,isocyanatoethane, 1-isocyanatopropane, 1-isocyanatobutane,1-isocyanatopentane, 1-isocyanatohexane, 1-isocyanatoheptane,1-isocyanatooctane, 1-isocyanatononane, 1-isocyanatodecane,isocyanatocyclohexane, methoxycarbonyl isocyanate, ethoxycarbonylisocyanate, propoxycarbonyl isocyanate, butoxycarbonyl isocyanate,methoxysulfonyl isocyanate, ethoxysulfonyl isocyanate, propoxysulfonylisocyanate, butoxysulfonyl isocyanate, fluorosulfonyl isocyanate, methylisocyanate, butyl isocyanate, phenyl isocyanate, 2-isocyanatoethylacrylate, 2-isocyanatoethyl methacrylate, and ethyl isocyanate.

Specific examples of the diisocyanates include 1,4-diisocyanatobutane,1,5-diisocyanatopentane, 1,6-diisocyanatohexane,1,7-diisocyanatoheptane, 1,8-diisocyanatooctane, 1,9-diisocyanatononane,1,10-diisocyanatodecane, 1,3-diisocyanatopropene,1,4-diisocyanato-2-butene, 1,4-diisocyanato-2-fluorobutane,1,4-diisocyanato-2,3-difluorobutane, 1,5-diisocyanato-2-pentene,1,5-diisocyanato-2-methylpentane, 1,6-diisocyanato-2-hexene,1,6-diisocyanato-3-hexene, 1,6-diisocyanato-3-fluorohexane,1,6-diisocyanato-3,4-difluorohexane, toluene diisocyanate, xylenediisocyanate, tolylene diisocyanate,1,2-bis(isocyanatomethyl)cyclohexane,1,3-bis(isocyanatomethyl)cyclohexane,1,4-bis(isocyanatomethyl)cyclohexane, 1,2-diisocyanatocyclohexane,1,3-diisocyanatocyclohexane, 1,4-diisocyanatocyclohexane,dicyclohexylmethane-1,1′-diisocyanate,dicyclohexylmethane-2,2′-diisocyanate,dicyclohexylmethane-3,3′-diisocyanate,dicyclohexylmethane-4,4′-diisocyanate, isophorone diisocyanate,bicyclo[2.2.1]heptane-2,5-diylbis(methyl=isocyanate),bicyclo[2.2.1]heptane-2,6-diylbis(methyl=isocyanate),2,4,4-trimethylhexamethylene diisocyanate, 2,2,4-trimethylhexamethylenediisocyanate, hexamethylene diisocyanate, 1,4-phenylene diisocyanate,octamethylene diisocyanate, and tetramethylene diisocyanate.

Specific examples of the triisocyanates include1,6,11-triisocyanatoundecane, 4-isocyanatomethyl-1,8-octamethylenediisocyanate, 1,3,5-triisocyanatomethylbenzene,1,3,5-tris(6-isocyanatohex-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,and 4-(isocyanatomethyl)octamethylene=diisocyanate.

In order to enable industrially easy availability and cause low cost inproduction of an electrolyte solution, preferred among these are1,6-diisocyanatohexane, 1,3-bis(isocyanatomethyl)cyclohexane,1,3,5-tris(6-isocyanatohex-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,2,4,4-trimethylhexamethylene diisocyanate, and2,2,4-trimethylhexamethylene diisocyanate. From the technical viewpoint,they can contribute to formation of a stable film-shaped structure andcan therefore more suitably be used.

The isocyanate may be present in any amount that does not significantlyimpair the effects of the invention. The amount is preferably, but notlimited to, 0.001% by mass or more and 1.0% by mass or less relative tothe electrolyte solution. The isocyanate in an amount of not smallerthan this lower limit can give a sufficient effect of improving thecycle characteristics to a non-aqueous electrolyte secondary battery.The isocyanate in an amount of not larger than this upper limit caneliminate an initial increase in resistance of a non-aqueous electrolytesecondary battery. The amount of the isocyanate is more preferably 0.01%by mass or more, still more preferably 0.1% by mass or more,particularly preferably 0.2% by mass or more, while more preferably 0.8%by mass or less, still more preferably 0.7% by mass or less,particularly preferably 0.6% by mass or less.

The electrolyte solution of the invention may contain a cyclicsulfonate. The cyclic sulfonate may be any cyclic sulfonate. Examples ofthe cyclic sulfonate include a saturated cyclic sulfonate, anunsaturated cyclic sulfonate, a saturated cyclic disulfonate, and anunsaturated cyclic disulfonate.

Specific examples of the saturated cyclic sulfonate include1,3-propanesultone, 1-fluoro-1,3-propanesultone,2-fluoro-1,3-propanesultone, 3-fluoro-1,3-propanesultone,1-methyl-1,3-propanesultone, 2-methyl-1,3-propanesultone,3-methyl-1,3-propanesultone, 1,3-butanesultone, 1,4-butanesultone,1-fluoro-1,4-butanesultone, 2-fluoro-1,4-butanesultone,3-fluoro-1,4-butanesultone, 4-fluoro-1,4-butanesultone,1-methyl-1,4-butanesultone, 2-methyl-1,4-butanesultone,3-methyl-1,4-butanesultone, 4-methyl-1,4-butanesultone, and2,4-butanesultone.

Specific examples of the unsaturated cyclic sulfonate include1-propene-1,3-sultone, 2-propene-1,3-sultone,1-fluoro-1-propene-1,3-sultone, 2-fluoro-1-propene-1,3-sultone,3-fluoro-1-propene-1,3-sultone, 1-fluoro-2-propene-1,3-sultone,2-fluoro-2-propene-1,3-sultone, 3-fluoro-2-propene-1,3-sultone,1-methyl-1-propene-1,3-sultone, 2-methyl-1-propene-1,3-sultone,3-methyl-1-propene-1,3-sultone, 1-methyl-2-propene-1,3-sultone,2-methyl-2-propene-1,3-sultone, 3-methyl-2-propene-1,3-sultone,1-butene-1,4-sultone, 2-butene-1,4-sultone, 3-butene-1,4-sultone,1-fluoro-1-butene-1,4-sultone, 2-fluoro-1-butene-1,4-sultone,3-fluoro-1-butene-1,4-sultone, 4-fluoro-1-butene-1,4-sultone,1-fluoro-2-butene-1,4-sultone, 2-fluoro-2-butene-1,4-sultone,3-fluoro-2-butene-1,4-sultone, 4-fluoro-2-butene-1,4-sultone,1,3-propenesultone, 1-fluoro-3-butene-1,4-sultone,2-fluoro-3-butene-1,4-sultone, 3-fluoro-3-butene-1,4-sultone,4-fluoro-3-butene-1,4-sultone, 1-methyl-1-butene-1,4-sultone,2-methyl-1-butene-1,4-sultone, 3-methyl-1-butene-1,4-sultone,4-methyl-1-butene-1,4-sultone, 1-methyl-2-butene-1,4-sultone,2-methyl-2-butene-1,4-sultone, 3-methyl-2-butene-1,4-sultone,4-methyl-2-butene-1,4-sultone, 1-methyl-3-butene-1,4-sultone,2-methyl-3-butene-1,4-sultone, 3-methyl-3-butene-1,4-sultone, and4-methyl-3-butene-14-sultone.

In order to enable easy availability and contribute to formation of astable film-shaped structure, more preferred among these are1,3-propanesultone, l-fluoro-1,3-propanesultone,2-fluoro-1,3-propanesultone, 3-fluoro-1,3-propanesultone, and1-propene-1,3-sultone. The cyclic sulfonate may be in any amount thatdoes not significantly impair the effects of the invention. The amountis preferably, but not limited to, 0.001% by mass or more and 3.0% bymass or less relative to the electrolyte solution.

The cyclic sulfonate in an amount of not smaller than this lower limitcan give a sufficient effect of improving the cycle characteristics to anon-aqueous electrolyte secondary battery. The cyclic sulfonate in anamount of not larger than this upper limit can eliminate an increase inthe cost of producing a non-aqueous electrolyte secondary battery. Theamount of the cyclic sulfonate is more preferably 0.01% by mass or more,still more preferably 0.1% by mass or more, particularly preferably 0.2%by mass or more, while more preferably 2.5% by mass or less, still morepreferably 2.0% by mass or less, particularly preferably 1.8% by mass orless.

The electrolyte solution of the invention may further contain apolyethylene oxide that has a weight average molecular weight of 2000 to4000 and has —OH, —OCOOH, or —COOH at an end.

The presence of such a compound can improve the stability at theinterfaces with the respective electrodes, improving the characteristicsof an electrochemical device.

Examples of the polyethylene oxide include polyethylene oxide monool,polyethylene oxide carboxylate, polyethylene oxide diol, polyethyleneoxide dicarboxylate, polyethylene oxide triol, and polyethylene oxidetricarboxylate. One of these may be used alone or two or more thereofmay be used in combination.

In order to give better characteristics of an electrochemical device,preferred are a mixture of polyethylene oxide monool and polyethyleneoxide diol and a mixture of polyethylene carboxylate and polyethylenedicarboxylate.

The polyethylene oxide having too small a weight average molecularweight may be easily oxidatively decomposed. The weight averagemolecular weight is more preferably 3000 to 4000.

The weight average molecular weight can be determined by gel permeationchromatography (GPC) in polystyrene equivalent.

The polyethylene oxide is preferably present in an amount of 1×10⁻⁶ to1×10⁻² mol/kg in the electrolyte solution. Too large an amount of thepolyethylene oxide may cause poor characteristics of an electrochemicaldevice.

The amount of the polyethylene oxide is more preferably 5×10⁻⁶ mol/kg ormore.

The electrolyte solution of the invention may further contain, as anadditive, any of other components such as a fluorinated saturated cycliccarbonate, an unsaturated cyclic carbonate, an overcharge inhibitor, anda known different aid. This can reduce impairment of the characteristicsof an electrochemical device.

An example of the fluorinated saturated cyclic carbonate is theaforementioned compound represented by the formula (A). Specifically,fluoroethylene carbonate, difluoroethylene carbonate,monofluoromethylethylene carbonate, and trifluoromethylethylenecarbonate are preferred. One of these fluorinated saturated cycliccarbonates may be used alone or two or more thereof may be used incombination.

The fluorinated saturated cyclic carbonate is preferably present in anamount of 0.001 to 10% by mass, more preferably 0.01 to 5% by mass,still more preferably 0.1 to 3% by mass, relative to the electrolytesolution.

Examples of the unsaturated cyclic carbonate include vinylene carbonatecompounds, ethylene carbonate compounds substituted with a substituentthat contains an aromatic ring, a carbon-carbon double bond, or acarbon-carbon triple bond, phenyl carbonate compounds, vinyl carbonatecompounds, allyl carbonate compounds, and catechol carbonate compounds.

Examples of the vinylene carbonate compounds include vinylene carbonate,methylvinylene carbonate, 4,5-dimethylvinylene carbonate, phenylvinylenecarbonate, 4,5-diphenylvinylene carbonate, vinylvinylene carbonate,4,5-divinylvinylene carbonate, allylvinylene carbonate,4,5-diallylvinylene carbonate, 4-fluorovinylene carbonate,4-fluoro-5-methylvinylene carbonate, 4-fluoro-5-phenylvinylenecarbonate, 4-fluoro-5-vinylvinylene carbonate, 4-allyl-5-fluorovinylenecarbonate, ethynylethylene carbonate, propargylethylene carbonate,methylvinylene carbonate, and dimethylvinylene carbonate.

Specific examples of the ethylene carbonate compounds substituted with asubstituent that contains an aromatic ring, a carbon-carbon double bond,or a carbon-carbon triple bond include vinylethylene carbonate,4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,4-allyl-5-vinylethylene carbonate, ethynylethylene carbonate,4,5-diethynylethylene carbonate, 4-methyl-5-ethynylethylene carbonate,4-vinyl-5-ethynylethylene carbonate, 4-allyl-5-ethynylethylenecarbonate, phenylethylene carbonate, 4,5-diphenylethylene carbonate,4-phenyl-5-vinylethylene carbonate, 4-allyl-5-phenylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate,4-methyl-5-allylethylene carbonate, 4-methylene-1,3-dioxolan-2-one,4,5-di methylene-1,3-dioxolan-2-one, and 4-methyl-5-allylethylenecarbonate.

The unsaturated cyclic carbonate is preferably vinylene carbonate,methylvinylene carbonate, 4,5-dimethylvinylene carbonate, vinylvinylenecarbonate, 4,5-vinylvinylene carbonate, allylvinylene carbonate,4,5-diallylvinylene carbonate, vinylethylene carbonate,4,5-divinylethylene carbonate, 4-methyl-5-vinylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate,4-methyl-5-allylethylene carbonate, 4-allyl-5-vinylethylene carbonate,ethynylethylene carbonate, 4,5-diethynylethylene carbonate,4-methyl-5-ethynylethylene carbonate, and 4-vinyl-5-ethynylethylenecarbonate. In order to form a more stable interface protecting film,particularly preferred are vinylene carbonate, vinylethylene carbonate,and ethynylethylene carbonate, and most preferred is vinylene carbonate.

The unsaturated cyclic carbonate may have any molecular weight that doesnot significantly impair the effects of the invention. The molecularweight is preferably 50 or higher and 250 or lower. The unsaturatedcyclic carbonate having a molecular weight within this range can easilyensure its solubility in the electrolyte solution and can easily lead tosufficient achievement of the effects of the invention. The molecularweight of the unsaturated cyclic carbonate is more preferably 80 orhigher and 150 or lower.

The unsaturated cyclic carbonate may be produced by any productionmethod, and may be produced by a known method selected as appropriate.

One unsaturated cyclic carbonate may be used alone or two or morethereof may be used in any combination at any ratio.

The unsaturated cyclic carbonate may be present in any amount that doesnot significantly impair the effects of the invention. The amount of theunsaturated cyclic carbonate is preferably 0.001% by mass or more, morepreferably 0.01% by mass or more, still more preferably 0.1% by mass ormore, of 100% by mass of the electrolyte solution. The amount ispreferably 5% by mass or less, more preferably 4% by mass or less, stillmore preferably 3% by mass or less. The unsaturated cyclic carbonate inan amount within the above range allows an electrochemical devicecontaining the electrolyte solution to easily exhibit a sufficienteffect of improving the cycle characteristics, and can easily avoid asituation with impaired high-temperature storage characteristics,generation of a large amount of gas, and a reduced discharge capacityretention.

In addition to the aforementioned non-fluorinated unsaturated cycliccarbonates, a fluorinated unsaturated cyclic carbonate may also suitablybe used as an unsaturated cyclic carbonate.

The fluorinated unsaturated cyclic carbonate is a cyclic carbonatecontaining an unsaturated bond and a fluorine atom. The number offluorine atoms in the fluorinated unsaturated cyclic carbonate may beany number that is 1 or greater. The number of fluorine atoms is usually6 or smaller, preferably 4 or smaller, most preferably 1 or 2.

Examples of the fluorinated unsaturated cyclic carbonate includefluorinated vinylene carbonate derivatives and fluorinated ethylenecarbonate derivatives substituted with a substituent that contains anaromatic ring or a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate,4-fluoro-5-phenylvinylene carbonate, 4-allyl-5-fluorovinylene carbonate,and 4-fluoro-5-vinylvinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substitutedwith a substituent that contains an aromatic ring or a carbon-carbondouble bond include 4-fluoro-4-vinylethylene carbonate,4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinylethylene carbonate,4-fluoro-5-allylethylene carbonate, 4,4-difluoro-4-vinylethylenecarbonate, 4,4-difluoro-4-allylethylene carbonate,4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-allylethylenecarbonate, 4-fluoro-4,5-divinylethylene carbonate,4-fluoro-4,5-diallylethylene carbonate, 4,5-difluoro-4,5-divinylethylenecarbonate, 4,5-difluoro-4,5-diallylethylene carbonate,4-fluoro-4-phenylethylene carbonate, 4-fluoro-5-phenylethylenecarbonate, 4,4-difluoro-5-phenylethylene carbonate, and4,5-difluoro-4-phenylethylene carbonate.

In order to form a stable interface protecting film, more preferablyused as the fluorinated unsaturated cyclic carbonate are4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate,4-fluoro-5-vinylvinylene carbonate, 4-allyl-5-fluorovinylene carbonate,4-fluoro-4-vinylethylene carbonate, 4-fluoro-4-allylethylene carbonate,4-fluoro-5-vinylethylene carbonate, 4-fluoro-5-allylethylene carbonate,4,4-difluoro-4-vinylethylene carbonate, 4,4-difluoro-4-allylethylenecarbonate, 4,5-difluoro-4-vinylethylene carbonate,4,5-difluoro-4-allylethylene carbonate, 4-fluoro-4,5-divinylethylenecarbonate, 4-fluoro-4,5-diallylethylene carbonate,4,5-difluoro-4,5-divinylethylene carbonate, and4,5-difluoro-4,5-diallylethylene carbonate.

The fluorinated unsaturated cyclic carbonate may have any molecularweight that does not significantly impair the effects of the invention.The molecular weight is preferably 50 or higher and 500 or lower. Thefluorinated unsaturated cyclic carbonate having a molecular weightwithin this range can easily ensure the solubility of the fluorinatedunsaturated cyclic carbonate in the electrolyte solution.

The fluorinated unsaturated cyclic carbonate may be produced by anymethod, and may be produced by any known method selected as appropriate.The molecular weight is more preferably 100 or higher and 200 or lower.

One fluorinated unsaturated cyclic carbonate may be used alone or two ormore thereof may be used in any combination at any ratio. Thefluorinated unsaturated cyclic carbonate may be contained in any amountthat does not significantly impair the effects of the invention. Theamount of the fluorinated unsaturated cyclic carbonate is usuallypreferably 0.001% by mass or more, more preferably 0.01% by mass ormore, still more preferably 0.1% by mass or more, while preferably 5% bymass or less, more preferably 4% by mass or less, still more preferably3% by mass or less, of 100% by mass of the electrolyte solution. Thefluorinated unsaturated cyclic carbonate in an amount within this rangeallows an electrochemical device containing the electrolyte solution toexhibit an effect of sufficiently improving the cycle characteristicsand can easily avoid a situation with reduced high-temperature storagecharacteristics, generation of a large amount of gas, and a reduceddischarge capacity retention.

The electrolyte solution of the invention may contain a compoundcontaining a triple bond. This compound may be of any type as long as itcontains one or more triple bonds in the molecule.

Specific examples of the compound containing a triple bond include thefollowing compounds:

hydrocarbon compounds such as 1-penthyne, 2-penthyne, 1-hexyne,2-hexyne, 3-hexyne, 1-heptyne, 2-heptyne, 3-heptyne, 1-octyne, 2-octyne,3-octyne, 4-octyne, 1-nonyne, 2-nonyne, 3-nonyne, 4-nonyne, 1-dodecyne,2-dodecyne, 3-dodecyne, 4-dodecyne, 5-dodecyne, phenyl acetylene,1-phenyl-1-propyne, 1-phenyl-2-propyne, 1-phenyl-1-butyne,4-phenyl-1-butyne, 4-phenyl-1-butyne, 1-phenyl-1-penthyne,5-phenyl-1-penthyne, l-phenyl-1-hexyne, 6-phenyl-1-hexyne, diphenylacetylene, 4-ethynyl toluene, and dicyclohexyl acetylene;

monocarbonates such as 2-propynylmethyl carbonate, 2-propynylethylcarbonate, 2-propynylpropyl carbonate, 2-propynylbutyl carbonate,2-propynylphenyl carbonate, 2-propynylcyclohexyl carbonate,di-2-propynylcarbonate, 1-methyl-2-propynylmethyl carbonate,1,1-dimethyl-2-propynylmethyl carbonate, 2-butynylmethyl carbonate,3-butynylmethyl carbonate, 2-pentynylmethyl carbonate, 3-pentynylmethylcarbonate, and 4-pentynylmethyl carbonate; dicarbonates such as2-butyne-1,4-diol dimethyl dicarbonate, 2-butyne-1,4-diol diethyldicarbonate, 2-butyne-1,4-diol dipropyl dicarbonate, 2-butyne-1,4-dioldibutyl dicarbonate, 2-butyne-1,4-diol diphenyl dicarbonate, and2-butyne-1,4-diol dicyclohexyl dicarbonate;

monocarboxylates such as 2-propynyl acetate, 2-propynyl propionate,2-propynyl butyrate, 2-propynyl benzoate, 2-propynylcyclohexylcarboxylate, 1,1-dimethyl-2-propynyl acetate,1,1-dimethyl-2-propynyl propionate, 1,1-dimethyl-2-propynyl butyrate,1,1-dimethyl-2-propynyl benzoate, 1,1-dimethyl-2-propynylcyclohexylcarboxylate, 2-butynyl acetate, 3-butynyl acetate, 2-pentynylacetate, 3-pentynyl acetate, 4-pentynyl acetate, methyl acrylate, ethylacrylate, propyl acrylate, vinyl acrylate, 2-propenyl acrylate,2-butenyl acrylate, 3-butenyl acrylate, methyl methacrylate, ethylmethacrylate, propyl methacrylate, vinyl methacrylate, 2-propenylmethacrylate, 2-butenyl methacrylate, 3-butenyl methacrylate, methyl2-propynoate, ethyl 2-propynoate, propyl 2-propynoate, vinyl2-propynoate, 2-propenyl 2-propynoate, 2-butenyl 2-propynoate, 3-butenyl2-propynoate, methyl 2-butynoate, ethyl 2-butynoate, propyl 2-butynoate,vinyl 2-butynoate, 2-propenyl 2-butynoate, 2-butenyl 2-butynoate,3-butenyl 2-butynoate, methyl 3-butynoate, ethyl 3-butynoate, propyl3-butynoate, vinyl 3-butynoate, 2-propenyl 3-butynoate, 2-butenyl3-butynoate, 3-butenyl 3-butynoate, methyl 2-penthynoate, ethyl2-penthynoate, propyl 2-penthynoate, vinyl 2-penthynoate, 2-propenyl2-penthynoate, 2-butenyl 2-penthynoate, 3-butenyl 2-penthynoate, methyl3-penthynoate, ethyl 3-penthynoate, propyl 3-penthynoate, vinyl3-penthynoate, 2-propenyl 3-penthynoate, 2-butenyl 3-penthynoate,3-butenyl 3-penthynoate, methyl 4-penthynoate, ethyl 4-penthynoate,propyl 4-penthynoate, vinyl 4-penthynoate, 2-propenyl 4-penthynoate,2-butenyl 4-penthynoate, and 3-butenyl 4-penthynoate, fumarates, methyltrimethylacetate, and ethyl trimethylacetate;

dicarboxylates such as 2-butyne-1,4-diol diacetate, 2-butyne-1,4-dioldipropionate, 2-butyne-1,4-diol dibutyrate, 2-butyne-1,4-dioldibenzoate, 2-butyne-1,4-diol dicyclohexanecarboxylate,hexahydrobenzo[1,3,2]dioxathiolane-2-oxide (1,2-cyclohexane diol,2,2-dioxide-1,2-oxathiolan-4-yl acetate, and2,2-dioxide-1,2-oxathiolan-4-yl acetate;

oxalic acid diesters such as methyl 2-propynyl oxalate, ethyl 2-propynyloxalate, propyl 2-propynyl oxalate, 2-propynyl vinyl oxalate, allyl2-propynyl oxalate, di-2-propynyl oxalate, 2-butynyl methyl oxalate,2-butynyl ethyl oxalate, 2-butynyl propyl oxalate, 2-butynyl vinyloxalate, allyl 2-butynyl oxalate, di-2-butynyl oxalate, 3-butynyl methyloxalate, 3-butynyl ethyl oxalate, 3-butynyl propyl oxalate, 3-butynylvinyl oxalate, allyl 3-butynyl oxalate, and di-3-butynyl oxalate;

phosphine oxides such as methyl(2-propynyl) (vinyl)phosphine oxide,divinyl(2-propynyl)phosphine oxide, di(2-propynyl) (vinyl)phosphineoxide, di(2-propenyl)2(-propynyl)phosphine oxide, di(2-propynyl)(2-propenyl)phosphine oxide, di(3-butenyl) (2-propynyl)phosphine oxide,and di(2-propynyl)(3-butenyl)phosphine oxide;

phosphinates such as 2-propynyl methyl(2-propenyl)phosphinate,2-propynyl 2-butenyl(methyl)phosphinate, 2-propynyldi(2-propenyl)phosphinate, 2-propynyl di(3-butenyl)phosphinate,1,1-dimethyl-2-propynyl methyl(2-propenyl)phosphinate,1,1-dimethyl-2-propynyl 2-butenyl(methyl)phosphinate,1,1-dimethyl-2-propynyl di(2-propenyl)phosphinate,1,1-dimethyl-2-propynyl di(3-butenyl)phosphinate, 2-propenylmethyl(2-propynyl)phosphinate, 3-butenyl methyl(2-propynyl)phosphinate,2-propenyl di(2-propynyl)phosphinate, 3-butenyldi(2-propynyl)phosphinate, 2-propenyl 2-propynyl(2-propenyl)phosphinate,and 3-butenyl 2-propynyl(2-propenyl)phosphinate;

phosphonates such as methyl 2-propynyl 2-propenylphosphonate,methyl(2-propynyl) 2-butenylphosphonate, (2-propynyl) (2-propenyl)2-propenylphosphonate, (3-butenyl) (2-propynyl) 3-butenylphosphonate,(1,1-dimethyl-2-propynyl) (methyl) 2-propenylphosphonate,(1,1-dimethyl-2-propynyl) (methyl) 2-butenylphosphonate,(1,1-dimethyl-2-propynyl) (2-propenyl) 2-propenylphosphonate,(3-butenyl) (1,1-dimethyl-2-propynyl) 3-butenylphosphonate, (2-propynyl)(2-propenyl) methylphosphonate, (3-butenyl)(2-propynyl)methylphosphonate, (1,1-dimethyl-2-propynyl) (2-propenyl)methylphosphonate, (3-butenyl) (1,1-dimethyl-2-propynyl)methylphosphonate, (2-propynyl) (2-propenyl) ethylphosphonate,(3-butenyl) (2-propynyl) ethylphosphonate, (1,1-dimethyl-2-propynyl)(2-propenyl) ethylphosphonate, and (3-butenyl) (1,1-dimethyl-2-propynyl)ethylphosphonate; and

phosphates such as (methyl) (2-propenyl) (2-propynyl) phosphate, (ethyl)(2-propenyl) (2-propynyl) phosphate, (2-butenyl) (methyl) (2-propynyl)phosphate, (2-butenyl) (ethyl) (2-propynyl) phosphate,(1,1-dimethyl-2-propynyl) (methyl) (2-propenyl) phosphate,(1,1-dimethyl-2-propynyl) (ethyl) (2-propenyl) phosphate, (2-butenyl)(1,1-dimethyl-2-propynyl) (methyl) phosphate, and (2-butenyl)(ethyl)(1,1-dimethyl-2-propynyl) phosphate.

In order to more stably form a negative electrode film in theelectrolyte solution, preferred among these are compounds containing analkynyloxy group.

In order to improve the storage characteristics, particularly preferredare compounds such as 2-propynylmethyl carbonate, di-2-propynylcarbonate, 2-butyne-1,4-diol dimethyl dicarbonate, 2-propynyl acetate,2-butyne-1,4-diol diacetate, methyl 2-propynyl oxalate, anddi-2-propynyl oxalate.

One compound containing a triple bond may be used alone or two or morethereof may be used in any combination at any ratio. The compoundcontaining a triple bond may be present in any amount that does notsignificantly impair the effects of the invention relative to the wholeelectrolyte solution of the invention. The compound is usually containedat a concentration of 0.01% by mass or more, preferably 0.05% by mass ormore, more preferably 0.1% by mass or more, while usually 5% by mass orless, preferably 3% by mass or less, more preferably 1% by mass or less,relative to the electrolyte solution of the invention. The compoundsatisfying the above range can further improve the effects such asoutput characteristics, load characteristics, cycle characteristics, andhigh-temperature storage characteristics.

In order to effectively reduce burst or combustion of a battery in caseof overcharge, for example, of an electrochemical device containing theelectrolyte solution, the electrolyte solution of the invention maycontain an overcharge inhibitor.

Examples of the overcharge inhibitor include aromatic compounds,including unsubstituted or alkyl-substituted terphenyl derivatives suchas biphenyl, o-terphenyl, m-terphenyl, and p-terphenyl, partiallyhydrogenated products of unsubstituted or alkyl-substituted terphenylderivatives, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenylether, dibenzofuran, diphenyl cyclohexane,1,1,3-trimethyl-3-phenylindan, cyclopentylbenzene, cyclohexylbenzene,cumene, 1,3-diisopropylbenzene, 1,4-diisopropylbenzene, t-butylbenzene,t-amylbenzene, t-hexylbenzene, and anisole; partially fluorinatedproducts of the aromatic compounds such as 2-fluorobiphenyl,4-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene,o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene, fluorobenzene,fluorotoluene, and benzotrifluoride; fluorine-containing anisolecompounds such as 2,4-difluoroanisole, 2,5-difluoroanisole,1,6-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole;aromatic acetates such as 3-propylphenyl acetate, 2-ethylphenyl acetate,benzylphenyl acetate, methylphenyl acetate, benzyl acetate, andphenethylphenyl acetate; aromatic carbonates such as diphenyl carbonateand methylphenyl carbonate, toluene derivatives such as toluene andxylene, and unsubstituted or alkyl-substituted biphenyl derivatives suchas 2-methylbiphenyl, 3-methylbiphenyl, 4-methylbiphenyl, ando-cyclohexylbiphenyl. Preferred among these are aromatic compounds suchas biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl,cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, anddibenzofuran, diphenyl cyclohexane, 1,1,3-trimethyl-3-phenylindan,3-propylphenyl acetate, 2-ethylphenyl acetate, benzylphenyl acetate,methylphenyl acetate, benzyl acetate, diphenyl carbonate, andmethylphenyl carbonate. One of these compounds may be used alone or twoor more thereof may be used in combination. In order to achieve goodbalance between the overcharge inhibiting characteristics and thehigh-temperature storage characteristics with a combination use of twoor more thereof, preferred is a combination of cyclohexylbenzene andt-butylbenzene or t-amylbenzene, or a combination of at least oneoxygen-free aromatic compound selected from biphenyl, alkylbiphenyl,terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene,t-butylbenzene, t-amylbenzene, and the like and at least oneoxygen-containing aromatic compound selected from diphenyl ether,dibenzofuran, and the like.

The electrolyte solution used in the invention may contain a carboxylicanhydride other than the compound (2). Preferred is a compoundrepresented by the following formula (6). The carboxylic anhydride maybe produced by any method which may be selected from known methods asappropriate.

In the formula (6), R⁶¹ and R⁶² are each individually a hydrocarbongroup having a carbon number of 1 or greater and 15 or smaller andoptionally containing a substituent.

R⁶¹ and R⁶² each may be any monovalent hydrocarbon group. For example,each of them may be either an aliphatic hydrocarbon group or an aromatichydrocarbon group, or may be a bond of an aliphatic hydrocarbon groupand an aromatic hydrocarbon group. The aliphatic hydrocarbon group maybe a saturated hydrocarbon group and may contain an unsaturated bond(carbon-carbon double bond or carbon-carbon triple bond). The aliphatichydrocarbon group may be either acyclic or cyclic. In the case of anacyclic group, it may be either linear or branched. The group may be abond of an acyclic group and a cyclic group. R⁶¹ and R⁶² may be the sameas or different from each other.

When the hydrocarbon group for R⁶¹ and R⁶² contains a substituent, thesubstituent may be of any type as long as it is not beyond the scope ofthe invention. Examples thereof include halogen atoms such as a fluorineatom, a chlorine atom, a bromine atom, and an iodine atom. Preferred isa fluorine atom. Examples of the substituent other than the halogenatoms include substituents containing a functional group such as anester group, a cyano group, a carbonyl group, or an ether group.Preferred are a cyano group and a carbonyl group. The hydrocarbon groupfor R⁶¹ and R⁶² may contain only one of these substituents or maycontain two or more thereof. When two or more substituents arecontained, these substituents may be the same as or different from eachother.

The hydrocarbon group for R⁶¹ and R⁶² has a carbon number of usually 1or greater, while usually 15 or smaller, preferably 12 or smaller, morepreferably 10 or smaller, still more preferably 9 or smaller. When R¹and R² bind to each other to form a divalent hydrocarbon group, thedivalent hydrocarbon group has a carbon number of usually 1 or greater,while usually 15 or smaller, preferably 13 or smaller, more preferably10 or smaller, still more preferably 8 or smaller. When the hydrocarbongroup for R⁶¹ and R⁶² contains a substituent that contains a carbonatom, the carbon number of the whole R⁶¹ or R⁶² including thesubstituent preferably satisfies the above range.

Next, specific examples of the acid anhydride represented by the formula(6) are described. In the following examples, the term “analog” means anacid anhydride obtainable by replacing part of the structure of an acidanhydride mentioned as an example by another structure within the scopeof the invention. Examples thereof include dimers, trimers, andtetramers each composed of a plurality of acid anhydrides, structuralisomers such as those having a substituent that has the same carbonnumber but also has a branch, and those having a different site at whicha substituent binds to the acid anhydride.

First, specific examples of an acid anhydride in which R⁶¹ and R⁶² arethe same as each other are described.

Specific examples of an acid anhydride in which R⁶¹ and R⁶² are linearalkyl groups include acetic anhydride, propionic anhydride, butanoicanhydride, 2-methylpropionic anhydride, 2,2-dimethylpropionic anhydride,2-methylbutanoic anhydride, 3-methylbutanoic anhydride,2,2-dimethylbutanoic anhydride, 2,3-dimethylbutanoic anhydride,3,3-dimethylbutanoic anhydride, 2,2,3-trimethylbutanoic anhydride,2,3,3-trimethylbutanoic anhydride, 2,2,3,3-tetramethylbutanoicanhydride, and 2-ethylbutanoic anhydride, and analogs thereof.

Specific examples of an acid anhydride in which R⁶¹ and R⁶² are cyclicalkyl groups include cyclopropanecarboxylic anhydride,cyclopentanecarboxylic anhydride, and cyclohexanecarboxylic anhydride,and analogs thereof.

Specific examples of an acid anhydride in which R⁶¹ and R⁶² are alkenylgroups include acrylic anhydride, 2-methylacrylic anhydride,3-methylacrylic anhydride, 2,3-dimethylacrylic anhydride,3,3-dimethylacrylic anhydride, 2,3,3-trimethylacrylic anhydride,2-phenylacrylic anhydride, 3-phenylacrylic anhydride,2,3-diphenylacrylic anhydride, 3,3-diphenylacrylic anhydride, 3-butenoicanhydride, 2-methyl-3-butenoic anhydride, 2,2-dimethyl-3-butenoicanhydride, 3-methyl-3-butenoic anhydride, 2-methyl-3-methyl-3-butenoicanhydride, 2,2-dimethyl-3-methyl-3-butenoic anhydride, 3-pentenoicanhydride, 4-pentenoic anhydride, 2-cyclopentenecarboxylic anhydride,3-cyclopentenecarboxylic anhydride, and 4-cyclopentenecarboxylicanhydride, and analogs thereof.

Specific examples of an acid anhydride in which R⁶¹ and R⁶² are alkynylgroups include propynoic anhydride, 3-phenylpropynoic anhydride,2-butynoic anhydride, 2-penthynoic anhydride, 3-butynoic anhydride,3-penthynoic anhydride, and 4-penthynoic anhydride, and analogs thereof.

Specific examples of an acid anhydride in which R⁶¹ and R⁶² are arylgroups include benzoic anhydride, 4-methylbenzoic anhydride,4-ethylbenzoic anhydride, 4-tert-butylbenzoic anhydride, 2-methylbenzoicanhydride, 2,4,6-trimethylbenzoic anhydride, 1-naphthalenecarboxylicanhydride, and 2-naphthalenecarboxylic anhydride, and analogs thereof.

Examples of an acid anhydride substituted with a fluorine atom aremainly listed below as examples of the acid anhydride in which R⁶¹ andR⁶² are substituted with a halogen atom. Acid anhydrides obtainable byreplacing any or all of the fluorine atoms thereof with a chlorine atom,a bromine atom, or an iodine atom are also included in the exemplarycompounds.

Examples of an acid anhydride in which R⁶¹ and R⁶² arehalogen-substituted linear alkyl groups include fluoroacetic anhydride,difluoroacetic anhydride, trifluoroacetic anhydride, 2-fluoropropionicanhydride, 2,2-difluoropropionic anhydride, 2,3-difluoropropionicanhydride, 2,2,3-trifluoropropionic anhydride, 2,3,3-trifluoropropionicanhydride, 2,2,3,3-tetrapropionic anhydride, 2,3,3,3-tetrapropionicanhydride, 3-fluoropropionic anhydride, 3,3-difluoropropionic anhydride,3,3,3-trifluoropropionic anhydride, and perfluoropropionic anhydride,and analogs thereof.

Examples of an acid anhydride in which R⁶¹ and R⁶² arehalogen-substituted cyclic alkyl groups include2-fluorocyclopentanecarboxylic anhydride, 3-fluorocyclopentanecarboxylicanhydride, and 4-fluorocyclopentanecarboxylic anhydride, and analogsthereof.

Examples of an acid anhydride in which R⁶¹ and R⁶² arehalogen-substituted alkenyl groups include 2-fluoroacrylic anhydride,3-fluoroacrylic anhydride, 2,3-difluoroacrylic anhydride,3,3-difluoroacrylic anhydride, 2,3,3-trifluoroacrylic anhydride,2-(trifluoromethyl)acrylic anhydride, 3-(trifluoromethyl)acrylicanhydride, 2,3-bis(trifluoromethyl)acrylic anhydride,2,3,3-tris(trifluoromethyl)acrylic anhydride, 2-(4-fluorophenyl)acrylicanhydride, 3-(4-fluorophenyl)acrylic anhydride,2,3-bis(4-fluorophenyl)acrylic anhydride, 3,3-bis(4-fluorophenyl)acrylicanhydride, 2-fluoro-3-butenoic anhydride, 2,2-difluoro-3-butenoicanhydride, 3-fluoro-2-butenoic anhydride, 4-fluoro-3-butenoic anhydride,3,4-difluoro-3-butenoic anhydride, and 3,3,4-trifluoro-3-butenoicanhydride, and analogs thereof.

Examples of an acid anhydride in which R⁶¹ and R⁶² arehalogen-substituted alkynyl groups include 3-fluoro-2-propynoicanhydride, 3-(4-fluorophenyl)-2-propynoic anhydride,3-(2,3,4,5,6-pentafluorophenyl)-2-propynoic anhydride,4-fluoro-2-butynoic anhydride, 4,4-difluoro-2-butynoic anhydride, and4,4,4-trifluoro-2-butynoic anhydride, and analogs thereof.

Examples of an acid anhydride in which R⁶¹ and R⁶² arehalogen-substituted aryl groups include 4-fluorobenzoic anhydride,2,3,4,5,6-pentafluorobenzoic anhydride, and 4-trifluoromethylbenzoicanhydride, and analogs thereof.

Examples of an acid anhydride in which R⁶¹ and R⁶² each contains asubstituent containing a functional group such as an ester, a nitrile, aketone, an ether, or the like include methoxyformic anhydride,ethoxyformic anhydride, methyloxalic anhydride, ethyloxalic anhydride,2-cyanoacetic anhydride, 2-oxopropionic anhydride, 3-oxobutanoicanhydride, 4-acetylbenzoic anhydride, methoxyacetic anhydride, and4-methoxybenzoic anhydride, and analogs thereof.

Then, specific examples of an acid anhydride in which R⁶¹ and R⁶² aredifferent from each other are described below.

R⁶¹ and R⁶² may be in any combination of those mentioned as examplesabove and analogs thereof. The following gives representative examples.

Examples of a combination of linear alkyl groups include aceticpropionic anhydride, acetic butanoic anhydride, butanoic propionicanhydride, and acetic 2-methylpropionic anhydride.

Examples of a combination of a linear alkyl group and a cyclic alkylgroup include acetic cyclopentanoic anhydride, acetic cyclohexanoicanhydride, and cyclopentanoic propionic anhydride.

Examples of a combination of a linear alkyl group and an alkenyl groupinclude acetic acrylic anhydride, acetic 3-methylacrylic anhydride,acetic 3-butenoic anhydride, and acrylic propionic anhydride.

Examples of a combination of a linear alkyl group and an alkynyl groupinclude acetic propynoic anhydride, acetic 2-butynoic anhydride, acetic3-butynoic anhydride, acetic 3-phenyl propynoic anhydride, and propionicpropynoic anhydride.

Examples of a combination of a linear alkyl group and an aryl groupinclude acetic benzoic anhydride, acetic 4-methylbenzoic anhydride,acetic 1-naphthalenecarboxylic anhydride, and benzoic propionicanhydride.

Examples of a combination of a linear alkyl group and a hydrocarbongroup containing a functional group include acetic fluoroaceticanhydride, acetic trifluoroacetic anhydride, acetic 4-fluorobenzoicanhydride, fluoroacetic propionic anhydride, acetic alkyloxalicanhydride, acetic 2-cyanoacetic anhydride, acetic 2-oxopropionicanhydride, acetic methoxyacetic anhydride, and methoxyacetic propionicanhydride.

Examples of a combination of cyclic alkyl groups include cyclopentanoiccyclohexanoic anhydride.

Examples of a combination of a cyclic alkyl group and an alkenyl groupinclude acrylic cyclopentanoic anhydride, 3-methylacrylic cyclopentanoicanhydride, 3-butenoic cyclopentanoic anhydride, and acryliccyclohexanoic anhydride.

Examples of a combination of a cyclic alkyl group and an alkynyl groupinclude propynoic cyclopentanoic anhydride, 2-butynoic cyclopentanoicanhydride, and propynoic cyclohexanoic anhydride.

Examples of a combination of a cyclic alkyl group and an aryl groupinclude benzoic cyclopentanoic anhydride, 4-methylbenzoic cyclopentanoicanhydride, and benzoic cyclohexanoic anhydride.

Examples of a combination of a cyclic alkyl group and a hydrocarbongroup containing a functional group include fluoroacetic cyclopentanoicanhydride, cyclopentanoic trifluoroacetic anhydride, cyclopentanoic2-cyanoacetic anhydride, cyclopentanoic methoxyacetic anhydride, andcyclohexanoic fluoroacetic anhydride.

Examples of a combination of alkenyl groups include acrylic2-methylacrylic anhydride, acrylic 3-methylacrylic anhydride, acrylic3-butenoic anhydride, and 2-methylacrylic 3-methylacrylic anhydride.

Examples of a combination of an alkenyl group and an alkynyl groupinclude acrylic propynoic anhydride, acrylic 2-butynoic anhydride, and2-methylacrylic propynoic anhydride.

Examples of a combination of an alkenyl group and an aryl group includeacrylic benzoic anhydride, acrylic 4-methylbenzoic anhydride, and2-methylacrylic benzoic anhydride.

Examples of a combination of an alkenyl group and a hydrocarbon groupcontaining a functional group include acrylic fluoroacetic anhydride,acrylic trifluoroacetic anhydride, acrylic 2-cyanoacetic anhydride,acrylic methoxyacetic anhydride, and 2-methylacrylic fluoroaceticanhydride.

Examples of a combination of alkynyl groups include propynoic 2-butynoicanhydride, propynoic 3-butynoic anhydride, and 2-butynoic 3-butynoicanhydride.

Examples of a combination of an alkynyl group and an aryl group includebenzoic propynoic anhydride, 4-methylbenzoic propynoic anhydride, andbenzoic 2-butynoic anhydride.

Examples of a combination of an alkynyl group and a hydrocarbon groupcontaining a functional group include propynoic fluoroacetic anhydride,propynoic trifluoroacetic anhydride, propynoic 2-cyanoacetic anhydride,propynoic methoxyacetic anhydride, and 2-butynoic fluoroaceticanhydride.

Examples of a combination of aryl groups include benzoic 4-methylbenzoicanhydride, benzoic 1-naphthalenecarboxylic anhydride, and4-methylbenzoic 1-naphthalenecarboxylic anhydride.

Examples of a combination of an aryl group and a hydrocarbon groupcontaining a functional group include benzoic fluoroacetic anhydride,benzoic trifluoroacetic anhydride, benzoic 2-cyanoacetic anhydride,benzoic methoxyacetic anhydride, and 4-methylbenzoic fluoroaceticanhydride.

Examples of a combination of hydrocarbon groups each containing afunctional group include fluoroacetic trifluoroacetic anhydride,fluoroacetic 2-cyanoacetic anhydride, fluoroacetic methoxyaceticanhydride, and trifluoroacetic 2-cyanoacetic anhydride.

Preferred among the acid anhydrides having an acyclic structure areacetic anhydride, propionic anhydride, 2-methylpropionic anhydride,cyclopentanecarboxylic anhydride, cyclohexanecarboxylic anhydride,acrylic anhydride, 2-methylacrylic anhydride, 3-methylacrylic anhydride,2,3-dimethylacrylic anhydride, 3,3-dimethylacrylic anhydride, 3-butenoicanhydride, 2-methyl-3-butenoic anhydride, propynoic anhydride,2-butynoic anhydride, benzoic anhydride, 2-methylbenzoic anhydride,4-methylbenzoic anhydride, 4-tert-butylbenzoic anhydride,trifluoroacetic anhydride, 3,3,3-trifluoropropionic anhydride,2-(trifluoromethyl)acrylic anhydride, 2-(4-fluorophenyl)acrylicanhydride, 4-fluorobenzoic anhydride, 2,3,4,5,6-pentafluorobenzoicanhydride, methoxyformic anhydride, and ethoxyformic anhydride. Morepreferred are acrylic anhydride, 2-methylacrylic anhydride,3-methylacrylic anhydride, benzoic anhydride, 2-methylbenzoic anhydride,4-methylbenzoic anhydride, 4-tert-butylbenzoic anhydride,4-fluorobenzoic anhydride, 2,3,4,5,6-pentafluorobenzoic anhydride,methoxyformic anhydride, and ethoxyformic anhydride.

These compounds are preferred because they can appropriately form a bondwith lithium oxalate to provide a film having excellent durability,thereby improving especially the charge and discharge ratecharacteristics after a durability test, input and outputcharacteristics, and impedance characteristics.

The carboxylic anhydride may have any molecular weight that does notsignificantly impair the effects of the invention. The molecular weightis usually 90 or higher, preferably 95 or higher, while usually 300 orlower, preferably 200 or lower. The carboxylic anhydride having amolecular weight within the above range can reduce an increase inviscosity of an electrolyte solution and can give a reasonable filmdensity, appropriately improving the durability.

The carboxylic anhydride may be formed by any production method whichmay be selected from known methods. One of the carboxylic anhydridesdescribed above alone may be contained in the non-aqueous electrolytesolution of the invention, or two or more thereof may be contained inany combination at any ratio.

The carboxylic anhydride may be contained in any amount that does notsignificantly impair the effects of the invention relative to theelectrolyte solution of the invention. The carboxylic anhydride isusually contained at a concentration of 0.01% by mass or more,preferably 0.1% by mass or more, while usually 5% by mass or less,preferably 3% by mass or less, relative to the electrolyte solution ofthe invention. The carboxylic anhydride in an amount within the aboverange can easily achieve an effect of improving the cyclecharacteristics and have good reactivity, easily improving the batterycharacteristics.

The electrolyte solution of the invention may further contain a knowndifferent aid. Examples of the different aid include carbonate compoundssuch as erythritan carbonate, spiro-bis-dimethylene carbonate, andmethoxyethyl-methyl carbonate; spiro compounds such as2,4,8,10-tetraoxaspiro[5.5]undecane and3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containingcompounds such as ethylene sulfite, methyl fluorosulfonate, ethylfluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate,busulfan, sulfolene, ethylene sulfate, vinylene sulfate, diphenylsulfone, N,N-dimethylmethanesulfonamide, N,N-diethylmethanesulfonamide,acetamide, N-methyl formamide, N,N-dimethyl formamide, N,N-dimethylacetamide, methyl vinyl sulfonate, ethyl vinyl sulfonate, allyl vinylsulfonate, propargyl vinyl sulfonate, methyl allyl sulfonate, ethylallyl sulfonate, allyl allyl sulfonate, propargyl allyl sulfonate,1,2-bis(vinylsulfonyloxy) ethane, propanedisulfonic anhydride,sulfobutyric anhydride, sulfobenzoic anhydride, sulfopropionicanhydride, ethanedisulfonic anhydride, methylene methanedisulfonate,2-propynyl methanesulfonate, pentene sulfite, pentafluorophenylmethanesulfonate, propylene sulfate, propylene sulfite, propane sultone,butylene sulfite, butane-2,3-diyl dimethanesulfonate, 2-butyne-1,4-diyldimethanesulfonate, 2-phenyl bicyclohexyl, 2-butyne-1,4-diyl diformate,2-propynyl vinyl sulfonate, bis(2-vinylsulfonylethyl)ether,5-vinyl-hexahydro-1,3,2-benzodioxathiol-2-oxide, 2-propynyl2-(methanesulfonyloxy)propionate, 5,5-dimethyl-1,2-oxathiolan-4-one2,2-dioxide, 3-sulfo-propionic anhydride, trimethylenemethanedisulfonate, 2-methyl tetrahydrofuran, trimethylenemethanedisulfonate, tetramethylene sulfoxide, dimethylenemethanedisulfonate, difluoroethyl methyl sulfone, divinyl sulfone,1,2-bis(vinylsulfonyl)ethane, methyl ethylenebisphosphonate, ethylethylenebisphosphonate, and ethylene sulfate; nitrogen-containingcompounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone,3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone,N-methylsuccinimide, nitromethane, nitroethane, thiophene 1-oxide, andethylene diamine; phosphorus-containing compounds such as trimethylphosphite, triethyl phosphite, triphenyl phosphite, trimethyl phosphate,triethyl phosphate, triphenyl phosphate, dimethyl methyl phosphonate,diethyl ethyl phosphonate, dimethyl vinyl phosphonate, diethyl vinylphosphonate, ethyl diethyl phosphonoacetate, methyl dimethylphosphinate, ethyl diethyl phosphinate, trimethylphosphine oxide,triethylphosphine oxide, bis(2,2-difluoroethyl)2,2,2-trifluoroethylphosphate, bis(2,2,3,3-tetrafluoropropyl)2,2,2-trifluoroethyl phosphate,bis(2,2,2-trifluoroethyl)methyl phosphate,bis(2,2,2-trifluoroethyl)ethyl phosphate,bis(2,2,2-trifluoroethyl)2,2-difluoroethyl phosphate,bis(2,2,2-trifluoroethyl)2,2,3,3-tetrafluoropropyl phosphate, tributylphosphate, tris(2,2,2-trifluoroethyl) phosphate,tris(1,1,1,3,3,3-hexafluoropropan-2-yl) phosphate, trioctyl phosphate,2-phenylphenyldimethyl phosphate, 2-phenylphenyldiethyl phosphate,(2,2,2-trifluoroethyl) (2,2,3,3-tetrafluoropropyl)methyl phosphate,phosphoric acid triester, methyl methylenebisphosphonate, ethylmethylenebisphosphonte, methyl 2-(dimethoxyphosphoryl)acetate, methyl2-(dimethylphosphoryl)acetate, methyl 2-(diethoxyphosphoryl)acetate,methyl 2-(diethylphosphoryl)acetate, methyl butylenebisphosphonate,ethyl butylenebisphosphonate, 2-propynyl 2-(dimethoxyphosphoryl)acetate,2-propynyl 2-(dimethylphosphoryl)acetate, 2-propynyl2-(diethoxyphosphoryl)acetate, 2-propynyl 2-(diethylphosphoryl)acetate,tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite,tris(triethylsilyl)phosphate, and tris(triethylsilyl)phosphite;hydrocarbon compounds such as heptane, octane, nonane, decane,cycloheptane, methyl formate, methyl 2-propynyl oxalate, 2-propynylmethacrylate, dimethyl malonate, malonic acid ester, pentane, benzene,furan, naphthalene, 2,5,8,11-tetraoxadodecane,2,5,8,11,14-pentaoxapentadecane, 3-pentanone, 2-phenyl bicyclohexyl,3-allyl succinic anhydride, ethoxymethoxyethane, trimethoxymethane, andcyclohexane; fluorine-containing aromatic compounds such asfluorobenzene, difluorobenzene, hexafluorobenzene, benzotrifluoride,monofluorobenzene, 1-fluoro-2-cyclohexylbenzene,1-fluoro-4-tert-butylbenzene, 1-fluoro-3-cyclohexylbenzene,1-fluoro-2-cyclohexylbenzene, and fluorinated biphenyl; silane compoundssuch as tris(trimethylsilyl) borate, tris(trimethoxysilyl) borate,tris(trimethylsilyl) phosphate, tris(trimethoxysilyl) phosphate,dimethoxyaluminoxytrimethoxysilane, diethoxyaluminoxytriethoxysilane,dipropoxyaluminoxytriethoxysilane, dibutoxyaluminoxytrimethoxysilane,dibutoxyaluminoxytriethoxysilane, titanium tetrakis(trimethylsiloxide),titanium tetrakis(triethylsiloxide)1, tris(trimethylsilyl) phosphite,trimethylsilyl polyphosphate, and tetramethylsilane; andoxygen-containing compounds such as dimethyl oxalate, dioxolane,dioxane, dimethyl ketone, diethyl ketone, diethyl oxalate,di(2-propynyl)oxalate, dimethyl succinate, di(2-propynyl) glutarate,2-propynyl formate, ethyl formate, glyme, ethyl monoglyme, ethylmethyloxalate, and ethyl formate. One of these compounds may be used alone ortwo or more thereof may be used in combination. These aids can improvethe capacity retention characteristics and the cycle characteristicsafter high-temperature storage.

Preferred among these as the different aid are phosphorus-containingcompounds, especially preferred is tris(trimethylsilyl)phosphate.

The different aid may be present in any amount that does notsignificantly impair the effects of the invention. The amount of thedifferent aid is preferably 0.01% by mass or more and 5% by mass or lessof 100% by mass of the electrolyte solution. The different aid in anamount within this range can easily sufficiently exhibit the effectsthereof and can easily avoid a situation with impairment of batterycharacteristics such as high-load discharge characteristics. The amountof the different aid is more preferably 0.1% by mass or more, still morepreferably 0.2% by mass or more, while more preferably 3% by mass orless, still more preferably 1% by mass or less.

The electrolyte solution of the invention may further contain as anadditive any of a cyclic carboxylate, an acyclic carboxylate, an ethercompound, a nitrogen-containing compound, a boron-containing compound,an organosilicon-containing compound, a fireproof agent (flameretardant), a surfactant, an additive for increasing the permittivity,an improver for cycle characteristics and rate characteristics, and asulfone-based compound to the extent that the effects of the inventionare not impaired.

Examples of the cyclic carboxylate include those having a carbon numberof 3 to 12 in total in the structural formula. Specific examples thereofinclude gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone,epsilon-caprolactone, and 3-methyl-γ-butyrolactone. In order to improvethe characteristics of an electrochemical device owing to improvement inthe degree of dissociation of lithium ions, particularly preferred isgamma-butyrolactone.

In general, the cyclic carboxylate as an additive is preferably presentin an amount of 0.1% by mass or more, more preferably 1% by mass ormore, of 100% by mass of the solvent. The cyclic carboxylate in anamount within this range can easily improve the electric conductivity ofthe electrolyte solution, improving the large-current dischargecharacteristics of an electrochemical device. The amount of the cycliccarboxylate is also preferably 10% by mass or less, more preferably 5%by mass or less. Such an upper limit may allow the electrolyte solutionto have a viscosity within an appropriate range, may make it possible toavoid a reduction in the electric conductivity, may reduce an increasein the resistance of the negative electrode, and may allow anelectrochemical device to have large-current discharge characteristicswithin a favorable range.

The cyclic carboxylate to be suitably used may also be a fluorinatedcyclic carboxylate (fluorine-containing lactone). Examples of thefluorine-containing lactone include fluorine-containing lactonesrepresented by the following formula (C):

wherein X¹⁵ to X²⁰ are the same as or different from each other, and areeach —H, —F, —Cl, —CH₃, or a fluorinated alkyl group; and at least oneselected from X¹⁵ to X²⁰ is a fluorinated alkyl group.

Examples of the fluorinated alkyl group for X¹⁵ to X²⁰ include —CFH₂,—CF₂H, —CF₃, —CH₂CF₃, —CF₂CF₃, —CH₂CF₂CF₃, and —CF(CF₃)₂. In order toachieve high oxidation resistance and an effect of improving the safety,—CH₂CF₃ and —CH₂CF₂CF₃ are preferred.

One of X¹⁵ to X²⁰ or a plurality thereof may be replaced by —H, —F, —Cl,—CH₃, or a fluorinated alkyl group only when at least one selected fromX¹⁵ to X²⁰ is a fluorinated alkyl group. In order to give goodsolubility of an electrolyte salt, the number of substituents ispreferably 1 to 3, more preferably 1 or 2.

The substitution of the fluorinated alkyl group may be at any of theabove sites. In order to give a good synthesizing yield, thesubstitution site is preferably X¹⁷ and/or X¹⁸. In particular, X¹⁷ orX¹⁸ is preferably a fluorinated alkyl group, especially —CH₂CF₃ or—CH₂CF₂CF₃. The substituent for X¹⁵ to X²⁰ other than the fluorinatedalkyl group is —H, —F, —Cl, or CH₃. In order to give good solubility ofan electrolyte salt, —H is preferred.

In addition to those represented by the above formula, thefluorine-containing lactone may also be a fluorine-containing lactonerepresented by the following formula (D):

wherein one of A or B is CX²²⁶X²²⁷ (where X²²⁶ and X²²⁷ are the same asor different from each other, and are each —H, —F, —Cl, —CF₃, —CH₃, oran alkylene group in which a hydrogen atom is optionally replaced by ahalogen atom and which optionally contains a hetero atom in the chain)and the other is an oxygen atom; Rf¹² is a fluorinated alkyl group orfluorinated alkoxy group optionally containing an ether bond; X²²¹ andX²²² are the same as or different from each other, and are each —H, —F,—Cl, —CF₃, or CH₃; X²²³ to X²²⁵ are the same as or different from eachother, and are each —H, —F, —Cl, or an alkyl group in which a hydrogenatom is optionally replaced by a halogen atom and which optionallycontains a hetero atom in the chain; and n=0 or 1.

A preferred example of the fluorine-containing lactone represented bythe formula (D) is a 5-membered ring structure represented by thefollowing formula (E):

(wherein A, B, Rf¹², X²²¹, X²²², and X²²³ are defined as in the formula(D)) because it can be easily synthesized and can have good chemicalstability. Further, in relation to the combination of A and B,fluorine-containing lactones represented by the following formula (F):

(wherein Rf¹², X²²¹, X²²², X²²³, X²²⁶, and X²²⁷ are defined as in theformula (D)) and fluorine-containing lactones represented by thefollowing formula (G):

(wherein Rf¹², X²²¹, X²²², X²²³, X²²⁶, and X²²⁷ are defined as in theformula (D)) may be mentioned.

In order to particularly give excellent characteristics such as highpermittivity and high withstand voltage, and to improve thecharacteristics of the electrolyte solution in the invention, forexample, to give good solubility of an electrolyte salt and to reducethe internal resistance well, those represented by the followingformulae:

may be mentioned.

The presence of a fluorinated cyclic carboxylate can lead to, forexample, effects of improving the ion conductivity, improving thesafety, and improving the stability at high temperature.

Examples of the acyclic carboxylate include those having a carbon numberof 3 to 7 in total in the structural formula thereof. Specific examplesthereof include methyl acetate, ethyl acetate, n-propyl acetate,isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate,methyl propionate, ethyl propionate, n-propyl propionate, isobutylpropionate, n-butyl propionate, methyl butyrate, isobutyl propionate,t-butyl propionate, methyl butyrate, ethyl butyrate, n-propyl butyrate,isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propylisobutyrate, and isopropyl isobutyrate.

In order to improve the ion conductivity owing to viscosity reduction,preferred among these are methyl acetate, ethyl acetate, n-propylacetate, n-butyl acetate, methyl propionate, ethyl propionate, n-propylpropionate, isopropyl propionate, methyl butyrate, and ethyl butyrate.

The ether compound is preferably a C2-C10 acyclic ether or a C3-C6cyclic ether.

Examples of the C2-C10 acyclic ether include dimethyl ether, diethylether, di-n-butyl ether, dimethoxymethane, methoxyethoxymethane,diethoxymethane, dimethoxyethane, methoxyethoxyethane, diethoxyethane,ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether,diethylene glycol, diethylene glycol dimethyl ether, pentaethyleneglycol, triethylene glycol dimethyl ether, triethylene glycol,tetraethylene glycol, tetraethylene glycol dimethyl ether, anddiisopropyl ether.

Further, the ether compound may also suitably be a fluorinated ether.

An example of the fluorinated ether is a fluorinated ether (I)represented by the following formula (I):

Rf³—O—Rf⁴  (I)

(wherein Rf³ and Rf⁴ are the same as or different from each other, andare each a C1-C10 alkyl group or a C1-C10 fluorinated alkyl group; andat least one selected from Rf³ and Rf⁴ is a fluorinated alkyl group).The presence of the fluorinated ether (I) allows the electrolytesolution to have improved incombustibility as well as improved stabilityand safety at high temperature under high voltage.

In the formula (I), at least one selected from Rf³ and Rf⁴ is a C1-C10fluorinated alkyl group. In order to allow the electrolyte solution tohave further improved incombustibility and further improved stabilityand safety at high temperature under high voltage, both Rf³ and Rf⁴ arepreferably C1-C10 fluorinated alkyl groups. In this case, Rf³ and Rf⁴may be the same as or different from each other.

Particularly preferably, Rf³ and Rf⁴ are the same as or different fromeach other, and Rf³ is a C3-C6 fluorinated alkyl group and Rf⁴ is aC2-C6 fluorinated alkyl group.

If the sum of the carbon numbers of Rf³ and Rf⁴ is too small, thefluorinated ether may have too low a boiling point. Too large a carbonnumber of Rf³ or Rf⁴ may cause low solubility of an electrolyte salt,may start to adversely affect the miscibility with other solvents, andmay cause high viscosity, resulting in poor rate characteristics. Inorder to achieve an excellent boiling point and rate characteristics,advantageously, the carbon number of Rf³ is 3 or 4 and the carbon numberof Rf⁴ is 2 or 3.

The fluorinated ether (I) preferably has a fluorine content of 40 to 75%by mass. The fluorinated ether (I) having a fluorine content within thisrange may lead to particularly excellent balance between thenon-flammability and the miscibility. The above range is also preferredfor good oxidation resistance and safety.

The lower limit of the fluorine content is more preferably 45% by mass,still more preferably 50% by mass, particularly preferably 55% by mass.The upper limit thereof is more preferably 70% by mass, still morepreferably 66% by mass.

The fluorine content of the fluorinated ether (I) is a value calculatedbased on the structural formula of the fluorinated ether (I) by thefollowing formula:

{(Number of fluorine atoms×19)/(Molecular weight of fluorinatedether(I))}×100(%).

Examples of Rf³ include CF₃CF₂CH₂—, CF₃CFHCF₂—, HCF₂CF₂CF₂—,HCF₂CF₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CFHCF₂CH₂—, HCF₂CF₂CF₂CF₂—,HCF₂CF₂CF₂CH₂—, HCF₂CF₂CH₂CH₂—, and HCF₂CF(CF₃) CH₂—. Examples of Rf⁴include —CH₂CF₂CF₃, —CF₂CFHCF₃, —CF₂CF₂CF₂H, —CH₂CF₂CF₂H, —CH₂CH₂CF₂CF₃,—CH₂CF₂CFHCF₃, —CF₂CF₂CF₂CF₂H, —CH₂CF₂CF₂CF₂H, —CH₂CH₂CF₂CF₂H,—CH₂CF(CF₃)CF₂H, —CF₂CF₂H, —CH₂CF₂H, and —CF₂CH₃.

Specific examples of the fluorinated ether (I) includeHCF₂CF₂CH₂OCF₂CF₂H, CF₃CF₂CH₂OCF₂CF₂H, HCF₂CF₂CH₂OCF₂CFHCF₃,CF₃CF₂CH₂OCF₂CFHCF₃, C₆F₁₃OCH₃, C₆F₁₃OC₂H₅, C₈F₁₇OCH₃, C₈F₁₇OC₂H₅,CF₃CFHCF₂CH(CH₃) OCF₂CFHCF₃, HCF₂CF₂OCH(C₂H₅) 2, HCF₂CF₂OC₄H₉,HCF₂CF₂OCH₂CH(C₂H₅)₂, and HCF₂CF₂OCH₂CH(CH₃) 2.

In particular, those having HCF₂— or CF₃CFH— at one or each end canprovide a fluorinated ether (I) having excellent polarizability and ahigh boiling point. The boiling point of the fluorinated ether (I) ispreferably 67° C. to 120° C., more preferably 80° C. or higher, stillmore preferably 90° C. or higher.

Such a fluorinated ether (I) may include one or two or more ofCF₃CH₂OCF₂CFHCF₃, CF₃CF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CFHCF₃,HCF₂CF₂CH₂OCH₂CF₂CF₂H, CF₃CFHCF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CF₂H,CF₃CF₂CH₂OCF₂CF₂H, and the like.

Advantageously, in order to achieve a high boiling point and goodmiscibility with other solvents and to give good solubility of anelectrolyte salt, the fluorinated ether (I) preferably include at leastone selected from the group consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boilingpoint: 106° C.), CF₃CF₂CH₂OCF₂CFHCF₃ (boiling point: 82° C.),HCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.), and CF₃CF₂CH₂OCF₂CF₂H(boiling point: 68° C.), more preferably at least one selected from thegroup consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.), andHCF₂CF₂CH₂OCF₂CF₂H (boiling point: 92° C.).

Examples of the C3-C6 cyclic ether include 1,2-dioxane, 1,3-dioxane,2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane,metaformaldehyde, 2-methyl-1,3-dioxolane, 1,3-dioxolane,4-methyl-1,3-dioxolane, 2-(trifluoroethyl)dioxolane,2,2,-bis(trifluoromethyl)-1,3-dioxolane, and fluorinated compoundsthereof. In order to achieve a high ability to solvate with lithium ionsand improve the degree of ion dissociation, preferred aredimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycoln-propyl ether, ethylene glycol di-n-butyl ether, diethylene glycoldimethyl ether, and crown ethers. In order to achieve low viscosity andto give a high ion conductivity, particularly preferred aredimethoxymethane, diethoxymethane, and ethoxymethoxymethane.

Examples of the nitrogen-containing compound include nitrile,fluorine-containing nitrile, carboxylic acid amide, fluorine-containingcarboxylic acid amide, sulfonic acid amide, fluorine-containing sulfonicacid amide, acetamide, and formamide. Also, 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazilidinone,1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide may be used. Thenitrile compounds represented by the formulae (1a), (1b), and (1c) arenot included in the above nitrogen-containing compounds.

Examples of the boron-containing compound include borates such astrimethyl borate and triethyl borate, boric acid ethers, and alkylborates.

Examples of the organosilicon-containing compound include (CH₃)₄—Si,(CH₃)₃—Si—Si(CH₃)₃, and silicone oil.

Examples of the fireproof agent (flame retardant) includeorganophosphates and phosphazene-based compounds. Examples of theorganophosphates include fluorine-containing alkyl phosphates,non-fluorine-containing alkyl phosphates, and aryl phosphates. In orderto achieve a flame retardant effect even in a small amount,fluorine-containing alkyl phosphates are particularly preferred.

Examples of the phosphazene-based compounds includemethoxypentafluorocyclotriphosphazene,phenoxypentafluorocyclotriphosphazene,dimethylaminopentafluorocyclotriphosphazene,diethylaminopentafluorocyclotriphosphazene,ethoxypentafluorocyclotriphosphazene, andethoxyheptafluorocyclotetraphosphazene.

Specific examples of the fluorine-containing alkyl phosphates includefluorine-containing dialkyl phosphates disclosed in JP H11-233141 A,cyclic alkyl phosphates disclosed in JP H11-283669 A, andfluorine-containing trialkyl phosphates.

Preferred examples of the fireproof agent (flame retardant) include(CH₃O)₃P═O, (CF₃CH₂O)₃P═O, (HCF₂CH₂O)₃P═O, (CF₃CF₂CH₂)₃P═O, and(HCF₂CF₂CH₂) 3P═O.

The surfactant may be any of cationic surfactants, anionic surfactants,nonionic surfactants, and amphoteric surfactants. In order to give goodcycle characteristics and rate characteristics, the surfactant ispreferably one containing a fluorine atom.

Preferred examples of such a surfactant containing a fluorine atominclude fluorine-containing carboxylic acid salts represented by thefollowing formula (30):

Rf⁵COO⁻M⁺  (30)

(wherein Rf⁵ is a C3-C10 fluorine-containing alkyl group optionallycontaining an ether bond; M⁺ is Li⁺, Na⁻, K⁺, or NHR′₃+, wherein R's arethe same as or different from each other, and are each H or a C1-C3alkyl group), and fluorine-containing sulfonic acid salts represented bythe following formula (40):

Rf⁶SO₃ ⁻M⁺  (40)

(wherein Rf⁶ is a C3-C10 fluorine-containing alkyl group optionallycontaining an ether bond; M⁺ is Li⁻, Na⁺, K⁺, or NHR′₃ ⁺, wherein R'sare the same as or different from each other, and are each H or a C1-C3alkyl group).

In order to reduce the surface tension of the electrolyte solutionwithout impairing the charge and discharge cycle characteristics, thesurfactant is preferably present in an amount of 0.01 to 2% by mass ofthe electrolyte solution.

Examples of the additive for increasing the permittivity includesulfolane, methylsulfolane, γ-butyrolactone, and γ-valerolactone.

Examples of the improver for cycle characteristics and ratecharacteristics include methyl acetate, ethyl acetate, tetrahydrofuran,and 1,4-dioxane.

The electrolyte solution of the invention may be combined with a polymermaterial and thereby formed into a gel-like (plasticized), gelelectrolyte solution.

Examples of such a polymer material include conventionally knownpolyethylene oxide and polypropylene oxide, and modified productsthereof (see JP H08-222270 A, JP 2002-100405 A); polyacrylate-basedpolymers, polyacrylonitrile, and fluororesins such as polyvinylidenefluoride and vinylidene fluoride-hexafluoropropylene copolymers (see JPH04-506726 T, JP H08-507407 T, JP H10-294131 A); and composites of anyof these fluororesins and any hydrocarbon resin (see JP H11-35765 A, JPH11-86630 A) In particular, polyvinylidene fluoride or a vinylidenefluoride-hexafluoropropylene copolymer is preferably used as a polymermaterial for a gel electrolyte.

The electrolyte solution of the invention may also contain an ionconductive compound disclosed in Japanese Patent Application No.2004-301934.

This ion conductive compound is an amorphous fluorine-containingpolyether compound having a fluorine-containing group at a side chainand is represented by the following formula (1-1):

A-(D)-B  (1-1)

wherein D is represented by the following formula (2-1):

-(D1)_(n)-(FAE)_(m)-((AE)_(p)-(Y)_(q)—  (2-1)

[wherein D1 is an ether unit containing a fluorine-containing ethergroup at a side chain and is represented by the following formula (2a):

(wherein Rf is a fluorine-containing ether group optionally containing acrosslinkable functional group; and R¹⁰ is a group or a bond that linksRf and the main chain);

FAE is an ether unit containing a fluorinated alkyl group at a sidechain and is represented by the following formula (2b):

(wherein Rfa is a hydrogen atom or a fluorinated alkyl group optionallycontaining a crosslinkable functional group; and R¹¹ is a group or abond that links Rfa and the main chain);

AE is an ether unit represented by the following formula (2c):

(wherein R¹³ is a hydrogen atom, an alkyl group optionally containing acrosslinkable functional group, an aliphatic cyclic hydrocarbon groupoptionally containing a crosslinkable functional group, or an aromatichydrocarbon group optionally containing a crosslinkable functionalgroup; and R¹² is a group or a bond that links R¹³ and the main chain);

Y is a unit containing at least one selected from the following formulae(2d-1) to (2d-3):

n is an integer of 0 to 200;

m is an integer of 0 to 200;

p is an integer of 0 to 10000;

q is an integer of 1 to 100;

n+m is not 0; and

the bonding order of D1, FAE, AE, and Y is not specified]; and

A and B are the same as or different from each other, and are each ahydrogen atom, an alkyl group optionally containing a fluorine atomand/or a crosslinkable functional group, a phenyl group optionallycontaining a fluorine atom and/or a crosslinkable functional group, a—COOH group, —OR (where R is a hydrogen atom or an alkyl groupoptionally containing a fluorine atom and/or a crosslinkable functionalgroup), an ester group, or a carbonate group, and when an end of D is anoxygen atom, A and B are each none of a —COOH group, —OR, an estergroup, and a carbonate group.

The electrolyte solution of the invention may contain a sulfone-basedcompound. Preferred as the sulfone-based compound are a C3-C6 cyclicsulfone and a C2-C6 acyclic sulfone. The number of sulfonyl groups inone molecule is preferably 1 or 2.

Examples of the cyclic sulfone include monosulfone compounds such astrimethylene sulfones, tetramethylene sulfones, and hexamethylenesulfones; disulfone compounds such as trimethylene disulfones,tetramethylene disulfones, and hexamethylene disulfones. In order togive good permittivity and viscosity, more preferred among these aretetramethylene sulfones, tetramethylene disulfones, hexamethylenesulfones, and hexamethylene disulfones, particularly preferred aretetramethylene sulfones (sulfolanes).

The sulfolanes are preferably sulfolane and/or sulfolane derivatives(hereinafter, also abbreviated as “sulfolanes” including sulfolane). Thesulfolane derivatives are preferably those in which one or more hydrogenatoms binding to any carbon atom constituting the sulfolane ring isreplaced by a fluorine atom or an alkyl group.

In order to achieve high ion conductivity and high input and output,preferred among these are 2-methylsulfolane, 3-methylsulfolane,2-fluorosulfolane, 3-fluorosulfolane, 2,2-difluorosulfolane,2,3-difluorosulfolane, 2,4-difluorosulfolane, 2,5-difluorosulfolane,3,4-difluorosulfolane, 2-fluoro-3-methylsulfolane,2-fluoro-2-methylsulfolane, 3-fluoro-3-methylsulfolane,3-fluoro-2-methylsulfolane, 4-fluoro-3-methylsulfolane,4-fluoro-2-methylsulfolane, 5-fluoro-3-methylsulfolane,5-fluoro-2-methylsulfolane, 2-fluoromethylsulfolane,3-fluoromethylsulfolane, 2-difluoromethylsulfolane,3-difluoromethylsulfolane, 2-trifluoromethylsulfolane,3-trifluoromethylsulfolane, 2-fluoro-3-(trifluoromethyl)sulfolane,3-fluoro-3-(trifluoromethyl)sulfolane,4-fluoro-3-(trifluoromethyl)sulfolane, 3-sulfolene,5-fluoro-3-(trifluoromethyl)sulfolane, and the like.

Examples of the acyclic sulfone include dimethyl sulfone, ethyl methylsulfone, diethyl sulfone, n-propyl methyl sulfone, n-propyl ethylsulfone, di-n-propyl sulfone, isopropyl methyl sulfone, isopropyl ethylsulfone, diisopropyl sulfone, n-butyl methyl sulfone, n-butyl ethylsulfone, t-butyl methyl sulfone, t-butyl ethyl sulfone, monofluoromethylmethyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methylsulfone, monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone,trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethylmonofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyltrifluoromethyl sulfone, perfluoroethyl methyl sulfone, ethyltrifluoroethyl sulfone, ethyl pentafluoroethyl sulfone,di(trifluoroethyl)sulfone, perfluorodiethyl sulfone,fluoromethyl-n-propyl sulfone, difluoromethyl-n-propyl sulfone,trifluoromethyl-n-propyl sulfone, fluoromethyl isopropyl sulfone,difluoromethyl isopropyl sulfone, trifluoromethyl isopropyl sulfone,trifluoroethyl-n-propyl sulfone, trifluoroethyl isopropyl sulfone,pentafluoroethyl-n-propyl sulfone, pentafluoroethyl isopropyl sulfone,trifluoroethyl-n-butyl sulfone, trifluoroethyl-t-butyl sulfone,pentafluoroethyl-n-butyl sulfone, and pentafluoroethyl-t-butyl sulfone.

In order to achieve high ion conductivity and high input and output,preferred among these are dimethyl sulfone, ethyl methyl sulfone,diethyl sulfone, n-propyl methyl sulfone, isopropyl methyl sulfone,n-butyl methyl sulfone, t-butyl methyl sulfone, monofluoromethyl methylsulfone, difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone,monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone,trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethylmonofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyltrifluoromethyl sulfone, ethyl trifluoroethyl sulfone, ethylpentafluoroethyl sulfone, trifluoromethyl-n-propyl sulfone,trifluoromethyl isopropyl sulfone, trifluoroethyl-n-butyl sulfone,trifluoroethyl-t-butyl sulfone, trifluoromethyl-n-butyl sulfone,trifluoromethyl-t-butyl sulfone, and the like.

The sulfone-based compound may be present in any amount that does notsignificantly impair the effects of the invention. The amount is usually0.3% by volume or more, preferably 0.5% by volume or more, morepreferably 1% by volume or more, while usually 40% by volume or less,preferably 35% by volume or less, more preferably 30% by volume or less,in 100% by volume of the solvent. The sulfone-based compound in anamount within the above range can easily achieve an effect of improvingthe cycle characteristics and the durability such as storagecharacteristics, can lead to an appropriate range of the viscosity of anon-aqueous electrolyte solution, can eliminate a reduction in electricconductivity, and can lead to appropriate ranges of the input and outputcharacteristics and charge and discharge rate characteristics of anon-aqueous electrolyte secondary battery.

The electrolyte solution of the invention may further contain adifferent additive as appropriate. Examples of the different additiveinclude metal oxides and glass.

In order to improve the output characteristics, the electrolyte solutionof the disclosure also preferably contains as an additive a compound (7)that is at least one selected from the group consisting of a lithiumfluorophosphate other than LiPF₆ and a lithium salt containing a S═Ogroup.

When the compound (7) is used as an additive, the above describedelectrolyte salt is preferably a compound other than the compound (7).

Examples of the lithium fluorophosphate include lithiummonofluorophosphate (LiPO₃F) and lithium difluorophosphate (LiPO₂F₂).

Examples of the lithium salt containing a S═O group include lithiummonofluorosulfonate (FSO₃Li), lithium methyl sulfate (CH₃OSO₃Li),lithium ethyl sulfate (C₂H₅OSO₃Li), and lithium 2,2,2-trifluoroethylsulfate.

Preferred among these as the compound (7) are LiPO₂F₂, FSO₃Li, andC₂H₅OSO₃Li.

The compound (7) is preferably present in an amount of 0.001 to 20% bymass, more preferably 0.01 to 15% by mass, still more preferably 0.1 to10% by mass, particularly preferably 0.1 to 7% by mass, relative to theelectrolyte solution.

The electrolyte solution of the invention preferably contains 5 to 200ppm of hydrogen fluoride (HF). The presence of HF can promote formationof a film of the aforementioned additive. Too small an amount of HFtends to impair the ability to form a film on the negative electrode,impairing the characteristics of an electrochemical device. Too large anamount of HF tends to impair the oxidation resistance of the electrolytesolution due to the influence by HF. The electrolyte solution of theinvention, even when containing HF in an amount within the above range,causes no reduction in capacity recovery of an electrochemical deviceafter high-temperature storage.

The amount of HF is more preferably 10 ppm or more, still morepreferably 20 ppm or more. The amount of HF is also more preferably 100ppm or less, still more preferably 80 ppm or less, particularlypreferably 50 ppm or less.

The amount of HF can be determined by neutralization titration.

The electrolyte solution of the invention is preferably prepared by anymethod using the aforementioned components.

The electrolyte solution of the invention can be suitably applied toelectrochemical devices such as lithium ion secondary batteries, lithiumion capacitors, hybrid capacitors, and electric double layer capacitors.Hereinafter, a non-aqueous electrolyte battery including the electrolytesolution of the invention is described.

The non-aqueous electrolyte battery can have a known structure,typically including positive and positive electrodes that can occludeand release ions (e.g., lithium ions) and the electrolyte solution ofthe invention. Such an electrochemical device including the electrolytesolution of the invention is also one aspect of the invention.

Examples of the electrochemical devices include lithium ion secondarybatteries, lithium ion capacitors, capacitors such as hybrid capacitorsand electric double-layer capacitors, radical batteries, solar cells, inparticular dye-sensitized solar cells, lithium ion primary batteries,fuel cells, various electrochemical sensors, electrochromic elements,electrochemical switching elements, aluminum electrolytic capacitors,and tantalum electrolytic capacitors. Preferred are lithium ionsecondary batteries, lithium ion capacitors, and electric double-layercapacitors.

A module including the electrochemical device is also one aspect of theinvention.

The invention also relates to a lithium ion secondary battery includingthe electrolyte solution of the invention.

The lithium ion secondary battery preferably includes a positiveelectrode, a negative electrode, and the above electrolyte solution.

<Positive Electrode>

The positive electrode includes a positive electrode active materiallayer containing a positive electrode active material and a currentcollector.

The positive electrode active material may be any material that canelectrochemically occlude and release lithium ions. Examples thereofinclude lithium-containing transition metal complex oxides,lithium-containing transition metal phosphoric acid compounds, sulfides,and conductive polymers. Preferred among these as the positive electrodeactive material are lithium-containing transition metal complex oxidesand lithium-containing transition metal phosphoric acid compounds.Particularly preferred is a lithium-containing transition metal complexoxide that generates high voltage.

The transition metal of the lithium-containing transition metal complexoxide is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specificexamples thereof include lithium-cobalt complex oxides such as LiCoO₂,lithium-nickel complex oxides such as LiNiO₂, lithium-manganese complexoxides such as LiMnO₂, LiMn₂O₄, and Li₂MnO₄, and those obtained bysubstituting some of transition metal atoms as main components of theselithium transition metal complex oxides with another element such as Na,K, B, F, Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Nb,Mo, Sn, or W. Specific examples of those obtained by substitutioninclude LiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.85) Co_(0.10)Al_(0.05)O₂,LiNi_(0.5) Co_(0.2)Mn_(0.3)O₂, LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂,LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂, LiNi_(0.45)Co_(0.10)Al_(0.45)O₂,LiMn_(1.8)Al_(0.2)O₄, and LiMn_(1.5)Ni_(0.5)O₄.

The lithium-containing transition metal complex oxide is preferably anyof LiMn_(1.5) Ni_(0.5)O₄, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, andLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ each of which has a high energy densityeven at high voltage.

The transition metal of the lithium-containing transition metalphosphoric acid compound is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, orthe like. Specific examples thereof include iron phosphates such asLiFePO₄, Li₃Fe₂(PO₄)₃, and LiFeP₂O₇, cobalt phosphates such as LiCoPO₄,and those obtained by substituting some of transition metal atoms asmain components of these lithium transition metal phosphoric acidcompounds with another element such as Al, Ti, V, Cr, Mn, Fe, Co, Li,Ni, Cu, Zn, Mg, Ga, Zr, Nb, or Si.

Examples of the lithium-containing transition metal complex oxideinclude

lithium-manganese spinel complex oxides represented by the formula:Li_(a)Mn_(2-b)M¹ _(b)O₄ (wherein 0.9≤a; 0≤b≤1.5; and M¹ is at least onemetal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn,Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge),

lithium-nickel complex oxides represented by the formula: LiNi_(1-c)M²_(c)O₂(wherein 0≤c≤0.5; and M² is at least one metal selected from thegroup consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr,B, Ga, In, Si, and Ge), and

lithium-cobalt complex oxides represented by the formula: LiCo_(1-d)M³_(d)O₂ (wherein 0≤d≤0.5; and M³ is at least one metal selected from thegroup consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr,B, Ga, In, Si, and Ge).

In order to provide a high-power lithium ion secondary battery having ahigh energy density, preferred is LiCoO₂, LiMnO₂, LiNiO₂, LiMn₂O₄,LiNi_(0.8)C_(0.15) Al_(0.05)O₂, or LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂.

Other examples of the positive electrode active material includeLiFePO₄, LiNi_(0.8)Co_(0.2)O₂, Li_(1.2)Fe_(0.4)Mn_(0.4)O₂,LiNi_(0.5)Mn_(0.5)O₂, and LiV₃O₆.

Examples of the sulfides include compounds having a 2D lamellarstructure such as TiS₂ and MoS₂, and chevrel compounds having a strong3D skeletal structure such as those represented by the formula:Me_(x)Mo₆S₈ (wherein Me is a transition metal such as Pb, Ag, and Cu).Examples thereof also include simple sulfur and organolithium sulfidesrepresented by LiS_(x).

Examples of the conductive polymers include p-doped conductive polymersand n-doped conductive polymers. Examples of the conductive polymersinclude polyacetylene-based polymers, polyphenylene-based polymers,heterocyclic polymers, ionic polymers, ladder-shaped polymers, andnetwork polymers.

In order to improve the continuous charge characteristics, the positiveelectrode active material preferably contains lithium phosphate. Lithiumphosphate may be used in any manner, and is preferably used in admixturewith the positive electrode active material. The lower limit of theamount of lithium phosphate used is preferably 0.1% by mass or more,more preferably 0.3% by mass or more, still more preferably 0.5% by massor more, relative to the sum of the amounts of the positive electrodeactive material and lithium phosphate. The upper limit thereof ispreferably 10% by mass or less, more preferably 8% by mass or less,still more preferably 5% by mass or less.

To a surface of the positive electrode active material may be attached asubstance having a composition different from the positive electrodeactive material. Examples of the substance attached to the surfaceinclude oxides such as aluminum oxide, silicon oxide, titanium oxide,zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimonyoxide, and bismuth oxide; sulfates such as lithium sulfate, sodiumsulfate, potassium sulfate, magnesium sulfate, calcium sulfate, andaluminum sulfate; carbonates such as lithium carbonate, calciumcarbonate, and magnesium carbonate; and carbon.

Such a substance may be attached to a surface of the positive electrodeactive material by, for example, a method of dissolving or suspendingthe substance in a solvent, impregnating the solution or suspension intothe positive electrode active material, and drying the impregnatedmaterial; a method of dissolving or suspending a precursor of thesubstance in a solvent, impregnating the solution or suspension into thepositive electrode active material, and heating the material and theprecursor to cause a reaction therebetween; or a method of adding thesubstance to a precursor of the positive electrode active material andsimultaneously sintering the materials. In the case of attaching carbon,for example, a carbonaceous material in the form of activated carbon maybe mechanically attached to the surface afterward.

For the amount of the substance attached to the surface in terms of themass relative to the amount of the positive electrode active material,the lower limit thereof is preferably 0.1 ppm or more, more preferably 1ppm or more, still more preferably 10 ppm or more, while the upper limitthereof is preferably 20% or less, more preferably 10% or less, stillmore preferably 5% or less. The substance attached to the surface canreduce oxidation of the electrolyte solution on the surface of thepositive electrode active material, improving the battery life. Toosmall an amount of the substance may fail to sufficiently provide thiseffect. Too large an amount thereof may hinder the entrance and exit oflithium ions, increasing the resistance.

Particles of the positive electrode active material may have any shapeconventionally used, such as a bulky shape, a polyhedral shape, aspherical shape, an ellipsoidal shape, a plate shape, a needle shape, ora pillar shape. The primary particles may agglomerate to form secondaryparticles.

The positive electrode active material has a tap density of preferably0.5 g/cm³ or higher, more preferably 0.8 g/cm³ or higher, still morepreferably 1.0 g/cm³ or higher. The positive electrode active materialhaving a tap density below the lower limit may cause an increased amountof a dispersion medium required and increased amounts of a conductivematerial and a binder required in formation of the positive electrodeactive material layer, as well as limitation on the packing fraction ofthe positive electrode active material in the positive electrode activematerial layer, resulting in limitation on the battery capacity. Acomplex oxide powder having a high tap density enables formation of apositive electrode active material layer with a high density. The tapdensity is preferably as high as possible and has no upper limit, ingeneral. Still, too high a tap density may cause diffusion of lithiumions in the positive electrode active material layer with theelectrolyte solution serving as a diffusion medium to function as arate-determining step, easily impairing the load characteristics. Thus,the upper limit of the tap density is preferably 4.0 g/cm³ or lower,more preferably 3.7 g/cm³ or lower, still more preferably 3.5 g/cm³ orlower.

In the invention, the tap density is determined as a powder packingdensity (tap density) g/cm³ when 5 to 10 g of the positive electrodeactive material powder is packed into a 10-ml glass graduated cylinderand the cylinder is tapped 200 times with a stroke of about 20 mm.

The particles of the positive electrode active material have a mediansize d50 (or a secondary particle size when the primary particlesagglomerate to form secondary particles) of preferably 0.3 μm orgreater, more preferably 0.5 μm or greater, still more preferably 0.8 μmor greater, most preferably 1.0 μm or greater, while preferably 30 μm orsmaller, more preferably 27 μm or smaller, still more preferably 25 μmor smaller, most preferably 22 μm or smaller. The particles having amedian size below the lower limit may fail to provide a product with ahigh tap density. The particles having a median size greater than theupper limit may cause prolonged diffusion of lithium in the particles,impairing the battery performance and generating streaks in formation ofthe positive electrode for a battery, i.e., when the active material andcomponents such as a conductive material and a binder are formed intoslurry by adding a solvent and the slurry is applied in the form of afilm, for example. Mixing two or more positive electrode activematerials having different median sizes d50 can further improve theeasiness of packing in formation of the positive electrode.

In the invention, the median size d50 is determined using a known laserdiffraction/scattering particle size distribution analyzer. In the caseof using LA-920 (Horiba, Ltd.) as the particle size distributionanalyzer, the dispersion medium used in the measurement is a 0.1% bymass sodium hexametaphosphate aqueous solution and the measurementrefractive index is set to 1.24 after 5-minute ultrasonic dispersion.

When the primary particles agglomerate to form secondary particles, theaverage primary particle size of the positive electrode active materialis preferably 0.05 μm or greater, more preferably 0.1 μm or greater,still more preferably 0.2 μm or greater. The upper limit thereof ispreferably 5 μm or smaller, more preferably 4 μm or smaller, still morepreferably 3 μm or smaller, most preferably 2 μm or smaller. The primaryparticles having an average primary particle size greater than the upperlimit may have difficulty in forming spherical secondary particles,adversely affecting the powder packing. Further, such primary particlesmay have a greatly reduced specific surface area, highly possiblyimpairing the battery performance such as output characteristics. Incontrast, the primary particles having an average primary particle sizebelow the lower limit may usually be insufficiently grown crystals,causing poor charge and discharge reversibility, for example.

In the invention, the primary particle size is measured by scanningelectron microscopic (SEM) observation. Specifically, the primaryparticle size is determined as follows. A photograph at a magnificationof 10000× is first taken. Any 50 primary particles are selected and themaximum length between the left and right boundary lines of each primaryparticle is measured along the horizontal line. Then, the average valueof the maximum lengths is calculated, which is defined as the primaryparticle size.

The positive electrode active material has a BET specific surface areaof preferably 0.1 m²/g or larger, more preferably 0.2 m²/g or larger,still more preferably 0.3 m²/g or larger. The upper limit thereof ispreferably 50 m²/g or smaller, more preferably 40 m²/g or smaller, stillmore preferably 30 m²/g or smaller. The positive electrode activematerial having a BET specific surface area smaller than the above rangemay easily impair the battery performance. The positive electrode activematerial having a BET specific surface area larger than the above rangemay less easily have an increased tap density, easily causing adifficulty in applying the material in formation of the positiveelectrode active material layer.

In the invention, the BET specific surface area is defined by a valuedetermined by single point BET nitrogen adsorption utilizing a gas flowmethod using a surface area analyzer (e.g., fully automatic surface areameasurement device, Ohkura Riken Co., Ltd.), a sample pre-dried innitrogen stream at 150° C. for 30 minutes, and a nitrogen-helium gasmixture with the nitrogen pressure relative to the atmospheric pressurebeing accurately adjusted to 0.3.

When the lithium ion secondary battery of the invention is used as alarge-size lithium ion secondary battery for hybrid vehicles ordistributed generation, it needs to achieve high output. Thus, theparticles of the positive electrode active material preferably mainlycomposed of secondary particles.

The particles of the positive electrode active material preferablyinclude 0.5 to 7.0% by volume of fine particles having an averagesecondary particle size of 40 μm or smaller and having an averageprimary particle size of 1 μm or smaller. The presence of fine particleshaving an average primary particle size of 1 μm or smaller enlarges thecontact area with the electrolyte solution and enables more rapiddiffusion of lithium ions between the electrode and the electrolytesolution, improving the output performance of the battery.

The positive electrode active material may be produced by any usualmethod of producing an inorganic compound. In particular, a spherical orellipsoidal active material can be produced by various methods. Forexample, a material substance of transition metal is dissolved orcrushed and dispersed in a solvent such as water, and the pH of thesolution or dispersion is adjusted under stirring to form a sphericalprecursor. The precursor is recovered and, if necessary, dried. Then, aLi source such as LiOH, Li₂CO₃, or LiNO₃ is added thereto and themixture is sintered at high temperature, thereby providing an activematerial.

In production of the positive electrode, one of the aforementionedpositive electrode active materials may be used alone or two or morethereof having different compositions may be used in any combination atany ratio. Preferred examples of the combination in this case include acombination of LiCoO₂ and LiMn₂O₄ in which part of Mn may optionally bereplaced by a different transition metal (e.g.,LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂), and a combination with LiCoO₂ in whichpart of Co may optionally be replaced by a different transition metal.

In order to achieve a high battery capacity, the amount of the positiveelectrode active material is preferably 50 to 99.5% by mass, morepreferably 80 to 99% by mass, of the positive electrode mixture. Theamount of the positive electrode active material in the positiveelectrode active material layer is preferably 80% by mass or more, morepreferably 82% by mass or more, particularly preferably 84% by mass ormore. The upper limit thereof is preferably 99% by mass or less, morepreferably 98% by mass or less. Too small an amount of the positiveelectrode active material in the positive electrode active materiallayer may cause an insufficient electric capacity. In contrast, toolarge an amount thereof may cause insufficient strength of the positiveelectrode.

The positive electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

The binder may be any material that is safe against a solvent to be usedin production of the electrode and the electrolyte solution. Examplesthereof include resin polymers such as polyethylene, polypropylene,polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide,chitosan, alginic acid, polyacrylic acid, polyimide, cellulose, andnitro cellulose; rubbery polymers such as SBR (styrene-butadienerubber), isoprene rubber, butadiene rubber, fluoroelastomers, NBR(acrylonitrile-butadiene rubber), and ethylene-propylene rubber;styrene-butadiene-styrene block copolymers and hydrogenated productsthereof; thermoplastic elastomeric polymers such as EPDM(ethylene-propylene-diene terpolymers),styrene-ethylene-butadiene-styrene copolymers, andstyrene-isoprene-styrene block copolymers and hydrogenated productsthereof; soft resin polymers such as syndiotactic-1,2-polybutadiene,polyvinyl acetate, ethylene-vinyl acetate copolymers, andpropylene-α-olefin copolymers; fluoropolymers such as polyvinylidenefluoride, polytetrafluoroethylene, vinylidene fluoride copolymer, andtetrafluoroethylene-ethylene copolymers; and polymer compositions havingion conductivity of alkali metal ions (especially, lithium ions). One ofthese may be used alone or two or more thereof may be used in anycombination at any ratio.

The amount of the binder, which is expressed as the proportion of thebinder in the positive electrode active material layer, is usually 0.1%by mass or more, preferably 1% by mass or more, more preferably 1.5% bymass or more. The proportion is also usually 80% by mass or less,preferably 60% by mass or less, still more preferably 40% by mass orless, most preferably 10% by mass or less. Too low a proportion of thebinder may fail to sufficiently hold the positive electrode activematerial and cause insufficient mechanical strength of the positiveelectrode, impairing the battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may causereduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, polyvinylpyrrolidone, andsalts thereof. One of these agents may be used alone or two or morethereof may be used in any combination at any ratio.

The proportion of the thickening agent relative to the active materialis usually 0.1% by mass or higher, preferably 0.2% by mass or higher,more preferably 0.3% by mass or higher, while usually 5% by mass orlower, preferably 3% by mass or lower, more preferably 2% by mass orlower. The thickening agent at a proportion lower than the above rangemay cause significantly poor easiness of application. The thickeningagent at a proportion higher than the above range may cause a lowproportion of the active material in the positive electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the positive electrode active materials.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite, including natural graphite andartificial graphite, carbon black, including acetylene black, Ketjenblack, channel black, furnace black, lamp black, and thermal black, andamorphous carbon, including needle coke, carbon nanotube, fullerene, andVGCF. One of these materials may be used alone or two or more thereofmay be used in any combination at any ratio. The conductive material isused in an amount of usually 0.01% by mass or more, preferably 0.1% bymass or more, more preferably 1% by mass or more, while usually 50% bymass or less, preferably 30% by mass or less, more preferably 15% bymass or less, in the positive electrode active material layer. Theconductive material in an amount less than the above range may causeinsufficient conductivity. In contrast, the conductive material in anamount more than the above range may cause a low battery capacity.

The solvent for forming slurry may be any solvent that can dissolve ordisperse therein the positive electrode active material, the conductivematerial, and the binder, as well as a thickening agent used asappropriate. The solvent may be either an aqueous solvent or an organicsolvent. Examples of the aqueous medium include water and solventmixtures of an alcohol and water. Examples of the organic medium includealiphatic hydrocarbons such as hexane; aromatic hydrocarbons such asbenzene, toluene, xylene, and methyl naphthalene; heterocyclic compoundssuch as quinoline and pyridine; ketones such as acetone, methyl ethylketone, and cyclohexanone; esters such as methyl acetate and methylacrylate; amines such as diethylene triamine andN,N-dimethylaminopropylamine; ethers such as diethyl ether, propyleneoxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone(NMP), dimethyl formamide, and dimethyl acetamide; and aprotic polarsolvents such as hexamethyl phospharamide and dimethyl sulfoxide.

Examples of the material of the current collector for a positiveelectrode include metal materials such as aluminum, titanium, tantalum,stainless steel, and nickel, and alloys thereof; and carbon materialssuch as carbon cloth and carbon paper. Preferred is any metal material,especially aluminum or an alloy thereof.

In the case of a metal material, the current collector may be in theform of metal foil, metal cylinder, metal coil, metal plate, metal film,expanded metal, punched metal, metal foam, or the like. In the case of acarbon material, it may be in the form of carbon plate, carbon film,carbon cylinder, or the like. Preferred among these is a metal film. Thefilm may be in the form of mesh, as appropriate. The film may have anythickness, and the thickness is usually 1 μm or greater, preferably 3 μmor greater, more preferably 5 μm or greater, while usually 1 mm orsmaller, preferably 100 μm or smaller, more preferably 50 μm or smaller.The film having a thickness smaller than the above range may haveinsufficient strength as a current collector. In contrast, the filmhaving a thickness greater than the above range may have poorhandleability.

In order to reduce the electric contact resistance between the currentcollector and the positive electrode active material layer, the currentcollector also preferably has a conductive aid applied on the surfacethereof. Examples of the conductive aid include carbon and noble metalssuch as gold, platinum, and silver.

The ratio between the thicknesses of the current collector and thepositive electrode active material layer may be any value, and the ratio{(thickness of positive electrode active material layer on one sideimmediately before injection of electrolyte solution)/(thickness ofcurrent collector)} is preferably 20 or lower, more preferably 15 orlower, most preferably 10 or lower. The ratio is also preferably 0.5 orhigher, more preferably 0.8 or higher, most preferably 1 or higher. Thecurrent collector and the positive electrode active material layershowing a ratio higher than the above range may cause the currentcollector to generate heat due to Joule heating duringhigh-current-density charge and discharge. The current collector and thepositive electrode active material layer showing a ratio lower than theabove range may cause an increased ratio by volume of the currentcollector to the positive electrode active material, reducing thebattery capacity.

The positive electrode may be produced by a usual method. An example ofthe production method is a method in which the positive electrode activematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likepositive electrode mixture, and then this mixture is applied to acurrent collector, dried, and pressed so as to be densified.

The densification may be achieved using a manual press or a roll press,for example. The density of the positive electrode active material layeris preferably 1.5 g/cm³ or higher, more preferably 2 g/cm³ or higher,still more preferably 2.2 g/cm³ or higher, while preferably 5 g/cm³ orlower, more preferably 4.5 g/cm³ or lower, still more preferably 4 g/cm³or lower. The positive electrode active material layer having a densityhigher than the above range may cause low permeability of theelectrolyte solution toward the vicinity of the interface between thecurrent collector and the active material, and poor charge and dischargecharacteristics particularly at a high current density, failing toprovide high output. The positive electrode active material layer havinga density lower than the above range may cause poor conductivity betweenthe active materials and increase the battery resistance, failing toprovide high output.

In order to improve the stability at high output and high temperature inthe case of using the electrolyte solution of the invention, the area ofthe positive electrode active material layer is preferably largerelative to the outer surface area of an external case of the battery.Specifically, the total area of the positive electrode is preferably 15times or more, more preferably 40 times or more, greater than thesurface area of the external case of the secondary battery. For closed,square-shaped cases, the outer surface area of an external case of thebattery herein means the total area calculated from the dimensions oflength, width, and thickness of the case portion into which apower-generating element is packed except for a protruding portion of aterminal. For closed, cylinder-like cases, the outer surface area of anexternal case of the battery herein means the geometric surface area ofan approximated cylinder of the case portion into which apower-generating element is packed except for a protruding portion of aterminal. The total area of the positive electrode herein means thegeometric surface area of the positive electrode mixture layer oppositeto a mixture layer including the negative electrode active material. Forstructures including a current collector foil and positive electrodemixture layers on both sides of the current collector, the total area ofthe positive electrode is the sum of the areas calculated on therespective sides.

The positive electrode plate may have any thickness. In order to achievea high capacity and high output, the lower limit of the thickness of themixture layer on one side of the current collector excluding thethickness of the base metal foil is preferably 10 μm or greater, morepreferably 20 μm or greater, while preferably 500 μm or smaller, morepreferably 450 μm or smaller.

To a surface of the positive electrode plate may be attached a substancehaving a composition different from the positive electrode plate.Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate;carbonates such as lithium carbonate, calcium carbonate, and magnesiumcarbonate; and carbon.

<Negative Electrode>

The negative electrode includes a negative electrode active materiallayer containing a negative electrode active material and a currentcollector.

The negative electrode material may be any one that canelectrochemically occlude and release lithium ions. Specific examplesthereof include carbon materials, alloyed materials, lithium-containingmetal complex oxide materials, and conductive polymers. One of these maybe used alone or two or more thereof may be used in any combination.

Examples of the negative electrode active material include carbonaceousmaterials that can occlude and release lithium such as pyrolysates oforganic matter under various pyrolysis conditions, artificial graphite,and natural graphite; metal oxide materials that can occlude and releaselithium such as tin oxide and silicon oxide; lithium metals; variouslithium alloys; and lithium-containing metal complex oxide materials.Two or more of these negative electrode active materials may be used inadmixture with each other.

The carbonaceous material that can occlude and release lithium ispreferably artificial graphite produced by high-temperature treatment ofeasily graphitizable pitch from various materials, purified naturalgraphite, or a material obtained by surface treatment on such graphitewith pitch or other organic matter and then carbonization of thesurface-treated graphite. In order to achieve a good balance between theinitial irreversible capacity and the high-current-density charge anddischarge characteristics, the carbonaceous material is more preferablyselected from carbonaceous materials obtained by heat-treating naturalgraphite, artificial graphite, artificial carbonaceous substances, orartificial graphite substances at 400° C. to 3200° C. once or more;carbonaceous materials which allow the negative electrode activematerial layer to include at least two or more carbonaceous mattershaving different crystallinities and/or have an interface between thecarbonaceous matters having the different crystallinities; andcarbonaceous materials which allow the negative electrode activematerial layer to have an interface between at least two or morecarbonaceous matters having different orientations. One of thesecarbonaceous materials may be used alone or two or more thereof may beused in any combination at any ratio.

Examples of the carbonaceous materials obtained by heat-treatingartificial carbonaceous substances or artificial graphite substances at400° C. to 3200° C. once or more include coal-based coke,petroleum-based coke, coal-based pitch, petroleum-based pitch, and thoseprepared by oxidizing these pitches; needle coke, pitch coke, and carbonmaterials prepared by partially graphitizing these cokes; pyrolysates oforganic matter such as furnace black, acetylene black, and pitch-basedcarbon fibers; carbonizable organic matter and carbides thereof; andsolutions prepared by dissolving carbonizable organic matter in alow-molecular-weight organic solvent such as benzene, toluene, xylene,quinoline, or n-hexane, and carbides thereof.

The metal material (excluding lithium-titanium complex oxides) to beused as the negative electrode active material may be any compound thatcan occlude and release lithium, and examples thereof include simplelithium, simple metals and alloys that constitute lithium alloys, andoxides, carbides, nitrides, silicides, sulfides, and phosphides thereof.The simple metals and alloys constituting lithium alloys are preferablymaterials containing any of metal and semi-metal elements in Groups 13and 14, more preferably simple metal of aluminum, silicon, and tin(hereinafter, referred to as “specific metal elements”), and alloys andcompounds containing any of these atoms. One of these materials may beused alone or two or more thereof may be used in any combination at anyratio.

Examples of the negative electrode active material containing at leastone atom selected from the specific metal elements include simple metalof any one specific metal element, alloys of two or more specific metalelements, alloys of one or two or more specific metal elements and oneor two or more other metal elements, compounds containing one or two ormore specific metal elements, and composite compounds such as oxides,carbides, nitrides, silicides, sulfides, and phosphides of thecompounds. Such a simple metal, alloy, or metal compound used as thenegative electrode active material can lead to a high-capacity battery.

Examples thereof further include compounds in which any of the abovecomposite compounds are complexly bonded with several elements such assimple metals, alloys, and nonmetal elements. Specifically, in the caseof silicon or tin, for example, an alloy of this element and a metalthat does not serve as a negative electrode may be used. In the case oftin, for example, a composite compound including a combination of 5 or 6elements, including tin, a metal (excluding silicon) that serves as anegative electrode, a metal that does not serve as a negative electrode,and a nonmetal element, may be used.

Specific examples thereof include simple Si, SiB₄, SiB₆, Mg₂Si, Ni₂Si,TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₆Si, FeSi₂, MnSi₂, NbSi₂,TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiO_(v) (0<v≤2), LiSiO,simple tin, SnSiO₃, LiSnO, Mg₂Sn, and SnO_(w) (0<w≤2).

Examples thereof further include composite materials of Si or Sn used asa first constitutional element, and second and third constitutionalelements. The second constitutional element is at least one selectedfrom cobalt, iron, magnesium, titanium, vanadium, chromium, manganese,nickel, copper, zinc, gallium, and zirconium, for example. The thirdconstitutional element is at least one selected from boron, carbon,aluminum, and phosphorus, for example.

In order to achieve a high battery capacity and excellent batterycharacteristics, the metal material is preferably simple silicon or tin(which may contain trace impurities), SiOv (0<v≤2), SnOw (0≤w≤2), aSi—Co—C composite material, a Si—Ni—C composite material, a Sn—Co—Ccomposite material, or a Sn—Ni—C composite material.

The lithium-containing metal complex oxide material to be used as thenegative electrode active material may be any material that can occludeand release lithium. In order to achieve good high-current-densitycharge and discharge characteristics, materials containing titanium andlithium are preferred, lithium-containing metal complex oxide materialscontaining titanium are more preferred, and complex oxides of lithiumand titanium (hereinafter, abbreviated as “lithium titanium complexoxides”) are still more preferred. In other words, use of aspinel-structured lithium titanium complex oxide in the negativeelectrode active material for an electrolyte battery is particularlypreferred because this can markedly reduce the output resistance.

Preferred examples of the lithium titanium complex oxides includecompounds represented by the following formula:

Li_(x)Ti_(y)M_(z)O₄

wherein M is at least one element selected from the group consisting ofNa, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

In order to achieve a good balance of the battery performance,particularly preferred among the above compositions are those satisfyingany of the following:

1.2≤x≤1.4,1.5≤y≤1.7,z=0  (i)

0.9≤x≤1.1,1.9≤y≤2.1,z=0  (ii)

0.7≤x≤0.9,2.1≤y≤2.3,z=0.  (iii)

Particularly preferred representative composition of the compound isLi_(4/3)Ti_(5/3)O₄ corresponding to the composition (i), Li₁Ti₂O₄corresponding to the composition (ii), and Li_(4/5)Ti_(11/5)O₄corresponding to the composition (iii). Preferred examples of thestructure satisfying Z≠0 include Li_(4/3)Ti_(4/3)Al_(1/3)O₄.

The negative electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

Examples of the binder include the same binders as those mentioned forthe positive electrode. The proportion of the binder is preferably 0.1%by mass or more, more preferably 0.5% by mass or more, particularlypreferably 0.6% by mass or more, while preferably 20% by mass or less,more preferably 15% by mass or less, still more preferably 10% by massor less, particularly preferably 8% by mass or less, relative to thenegative electrode active material. The binder at a proportion relativeto the negative electrode active material higher than the above rangemay lead to an increased proportion of the binder which fails tocontribute to the battery capacity, causing a low battery capacity. Thebinder at a proportion lower than the above range may cause loweredstrength of the negative electrode.

In particular, in the case of using a rubbery polymer typified by SBR asa main component, the proportion of the binder is usually 0.1% by massor more, preferably 0.5% by mass or more, more preferably 0.6% by massor more, while usually 5% by mass or less, preferably 3% by mass orless, more preferably 2% by mass or less, relative to the negativeelectrode active material. In the case of using a fluoropolymer typifiedby polyvinylidene fluoride as a main component, the proportion of thebinder is usually 1% by mass or more, preferably 2% by mass or more,more preferably 3% by mass or more, while usually 15% by mass or less,preferably 10% by mass or less, more preferably 8% by mass or less,relative to the negative electrode active material.

Examples of the thickening agent include the same thickening agents asthose mentioned for the positive electrode. The proportion of thethickening agent is usually 0.1% by mass or higher, preferably 0.5% bymass or higher, still more preferably 0.6% by mass or higher, whileusually 5% by mass or lower, preferably 3% by mass or lower, still morepreferably 2% by mass or lower, relative to the negative electrodeactive material. The thickening agent at a proportion relative to thenegative electrode active material lower than the above range may causesignificantly poor easiness of application. The thickening agent at aproportion higher than the above range may cause a small proportion ofthe negative electrode active material in the negative electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the negative electrode active materials.

Examples of the conductive material of the negative electrode includemetal materials such as copper and nickel; and carbon materials such asgraphite and carbon black.

The solvent for forming slurry may be any solvent that can dissolve ordisperse the negative electrode active material and the binder, as wellas a thickening agent and a conductive material used as appropriate. Thesolvent may be either an aqueous solvent or an organic solvent.

Examples of the aqueous solvent include water and alcohols. Examples ofthe organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethyl acetamide, methyl ethyl ketone, cyclohexanone,methyl acetate, methyl acrylate, diethyl triamine, N,N-dimethylaminopropyl amine, tetrahydrofuran (THF), toluene, acetone, diethylether, dimethyl acetamide, hexamethyl phospharamide, dimethyl sulfoxide,benzene, xylene, quinoline, pyridine, methyl naphthalene, and hexane.

Examples of the material of the current collector for a negativeelectrode include copper, nickel, and stainless steel. In order toeasily process the material into a film and to minimize the cost, copperfoil is preferred.

The current collector usually has a thickness of 1 μm or greater,preferably 5 μm or greater, while usually 100 μm or smaller, preferably50 μm or smaller. Too thick a negative electrode current collector maycause an excessive reduction in capacity of the whole battery, while toothin a current collector may be difficult to handle.

The negative electrode may be produced by a usual method. An example ofthe production method is a method in which the negative electrodematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likemixture, and then this mixture is applied to a current collector, dried,and pressed so as to be densified. In the case of using an alloyedmaterial, a thin film layer containing the above negative electrodeactive material (negative electrode active material layer) may beproduced by vapor deposition, sputtering, plating, or the like.

The electrode formed from the negative electrode active material mayhave any structure. The negative electrode active material existing onthe current collector preferably has a density of 1 g·cm⁻³ or higher,more preferably 1.2 g·cm⁻³ or higher, particularly preferably 1.3 g·cm⁻³or higher, while preferably 2.2 g·cm⁻³ or lower, more preferably 2.1g·cm⁻³ or lower, still more preferably 2.0 g·cm⁻³ or lower, particularlypreferably 1.9 g·cm⁻³ or lower. The negative electrode active materialexisting on the current collector having a density higher than the aboverange may cause destruction of the negative electrode active materialparticles, resulting in a high initial irreversible capacity and poorhigh-current-density charge and discharge characteristics due toreduction in permeability of the electrolyte solution toward thevicinity of the interface between the current collector and the negativeelectrode active material. The negative electrode active material havinga density below the above range may cause poor conductivity between thenegative electrode active materials, high battery resistance, and a lowcapacity per unit volume.

The thickness of the negative electrode plate is a design matter inaccordance with the positive electrode plate to be used, and may be anyvalue. The thickness of the mixture layer excluding the thickness of thebase metal foil is usually 15 μm or greater, preferably 20 μm orgreater, more preferably 30 μm or greater, while usually 300 μm orsmaller, preferably 280 μm or smaller, more preferably 250 μm orsmaller.

To a surface of the negative electrode plate may be attached a substancehaving a composition different from the negative electrode plate.Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; andcarbonates such as lithium carbonate, calcium carbonate, and magnesiumcarbonate.

<Separator>

The lithium ion secondary battery of the invention preferably furtherincludes a separator.

The separator may be formed from any known material and may have anyknown shape as long as the resulting separator is stable to theelectrolyte solution and is excellent in a liquid-retaining ability. Theseparator is preferably in the form of a porous sheet or a nonwovenfabric which is formed from a material stable to the electrolytesolution of the invention, such as resin, glass fiber, or inorganicmatter, and which has an excellent liquid-retaining ability.

Examples of the material of a resin or glass-fiber separator includepolyolefins such as polyethylene and polypropylene, aromatic polyamide,polytetrafluoroethylene, polyether sulfone, and glass filters. One ofthese materials may be used alone or two or more thereof may be used inany combination at any ratio, for example, in the form of apolypropylene/polyethylene bilayer film or apolypropylene/polyethylene/polypropylene trilayer film. In order toachieve good permeability of the electrolyte solution and a goodshut-down effect, the separator is preferably a porous sheet or anonwoven fabric formed from a polyolefin such as polyethylene orpolypropylene.

The separator may have any thickness, and the thickness is usually 1 μmor greater, preferably 5 μm or greater, more preferably 8 μm or greater,while usually 50 μm or smaller, preferably 40 μm or smaller, morepreferably 30 μm or smaller. The separator thinner than the above rangemay have poor insulation and mechanical strength. The separator thickerthan the above range may cause not only poor battery performance such aspoor rate characteristics but also a low energy density of the wholeelectrolyte battery.

The separator which is a porous one such as a porous sheet or a nonwovenfabric may have any porosity. The porosity is usually 20% or higher,preferably 35% or higher, more preferably 45′ or higher, while usually90% or lower, preferably 85% or lower, more preferably 75% or lower. Theseparator having a porosity lower than the above range tends to havehigh film resistance, causing poor rate characteristics. The separatorhaving a porosity higher than the above range tends to have lowmechanical strength, causing poor insulation.

The separator may also have any average pore size. The average pore sizeis usually 0.5 μm or smaller, preferably 0.2 μm or smaller, whileusually 0.05 μm or larger. The separator having an average pore sizelarger than the above range may easily cause short circuits. Theseparator having an average pore size smaller than the above range mayhave high film resistance, causing poor rate characteristics.

Examples of the inorganic matter include oxides such as alumina andsilicon dioxide, nitrides such as aluminum nitride and silicon nitride,and sulfates such as barium sulfate and calcium sulfate, each in theform of particles or fibers.

The separator is in the form of a thin film such as a nonwoven fabric, awoven fabric, or a microporous film. The thin film favorably has a poresize of 0.01 to 1 μm and a thickness of 5 to 50 μm. Instead of the aboveseparate thin film, the separator may have a structure in which acomposite porous layer containing particles of the above inorganicmatter is disposed on a surface of one or each of the positive andnegative electrodes using a resin binder. For example, alumina particleshaving a 90% particle size of smaller than 1 μm may be applied to therespective surfaces of the positive electrode with fluororesin used as abinder to form a porous layer.

<Battery Design>

The electrode group may be either a laminate structure including theabove positive and negative electrode plates with the above separator inbetween, or a wound structure including the above positive and negativeelectrode plates in spiral with the above separator in between. Theproportion of the volume of the electrode group in the battery internalvolume (hereinafter, referred to as an electrode group proportion) isusually 40% or higher, preferably 50% or higher, while usually 90% orlower, preferably 80% or lower.

The electrode group proportion lower than the above range may cause alow battery capacity. The electrode group proportion higher than theabove range may cause small void space in the battery. Thus, if thebattery temperature rises to high temperature and thereby the componentsswell and the liquid fraction of the electrolyte solution exhibits highvapor pressure to raise the internal pressure, the batterycharacteristics such as charge and discharge repeatability andhigh-temperature storageability may be impaired and a gas-releasingvalve for releasing the internal pressure toward the outside may beactuated.

The current collecting structure may be any structure. In order to moreeffectively improve the high-current-density charge and dischargeperformance by the electrolyte solution of the invention, the currentcollecting structure is preferably a structure which reduces theresistances at wiring portions and jointing portions. Such reduction ininternal resistance can particularly favorably lead to the effectsachieved with the electrolyte solution of the invention.

In an electrode group having the laminate structure, the metal coreportions of the respective electrode layers are preferably bundled andwelded to a terminal. If an electrode has a large area, the internalresistance is high. Thus, multiple terminals may preferably be disposedin the electrode so as to reduce the resistance. In an electrode grouphaving the wound structure, multiple lead structures may be disposed oneach of the positive electrode and the negative electrode and bundled toa terminal. This can reduce the internal resistance.

The external case may be made of any material that is stable to anelectrolyte solution to be used. Specific examples thereof includemetals such as nickel-plated steel plates, stainless steel, aluminum andaluminum alloys, and magnesium alloys, and a layered film (laminatefilm) of resin and aluminum foil. In order to reduce the weight, a metalsuch as aluminum or an aluminum alloy or a laminate film is favorablyused.

An external case made of metal may have a sealed-up structure formed bywelding the metal by laser welding, resistance welding, or ultrasonicwelding, or a caulking structure using the metal with a resin gasket inbetween. An external case made of a laminate film may have a sealed-upstructure formed by hot-melting resin layers. In order to improve thesealability, a resin which is different from the resin of the laminatefilm may be disposed between the resin layers. Especially, in the caseof forming a sealed-up structure by hot-melting the resin layers withcurrent collecting terminals in between, metal and resin are to bebonded. Thus, the resin to be disposed between the resin layers isfavorably a resin having a polar group or a modified resin having apolar group introduced therein.

The lithium ion secondary battery of the invention may have any shape,such as a cylindrical shape, a square shape, a laminate shape, a coinshape, or a large-size shape. The shapes and the structures of thepositive electrode, the negative electrode, and the separator may bechanged in accordance with the shape of the battery.

A module including the lithium ion secondary battery of the invention isalso one aspect of the invention.

In a preferred embodiment, the lithium ion secondary battery includes apositive electrode, a negative electrode, and the aforementionedelectrolyte solution, the positive electrode including a positiveelectrode current collector and a positive electrode active materiallayer containing a positive electrode active material, the positiveelectrode active material containing Mn. The lithium ion secondarybattery including a positive electrode active material layer thatcontains a positive electrode active material containing Mn can havemuch better high-temperature storage characteristics.

In order to provide a high-power lithium ion secondary battery having ahigh energy density, preferred as the positive electrode active materialcontaining Mn are LiMn_(1.5)Ni_(0.5)O₄, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂,and LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂.

The amount of the positive electrode active material in the positiveelectrode active material layer is preferably 80% by mass or more, morepreferably 82% by mass or more, particularly preferably 84% by mass ormore. The upper limit of the amount thereof is preferably 99% by mass orless, more preferably 98% by mass or less. Too small an amount of thepositive electrode active material in the positive electrode activematerial layer may lead to an insufficient electric capacity. Incontrast, too large an amount thereof may lead to insufficient strengthof the positive electrode.

The positive electrode active material layer may further contain aconductive material, a thickening agent, and a binder.

The binder may be any material that is safe against a solvent to be usedin production of electrodes and the electrolyte solution. Examplesthereof include polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polypropylene, SBR (styrene-butadiene rubber), isoprenerubber, butadiene rubber, ethylene-acrylic acid copolymers,ethylene-methacrylic acid copolymers, polyethylene terephthalate,polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, NBR (acrylonitrile-butadiene rubber), fluoroelastomer,ethylene-propylene rubber, styrene-butadiene-styrene block copolymersand hydrogenated products thereof, EPDM (ethylene-propylene-dieneterpolymers), styrene-ethylene-butadiene-ethylene copolymers,styrene-isoprene-styrene block copolymers and hydrogenated productsthereof, syndiotactic-1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymers, propylene-α-olefin copolymers,fluorinated polyvinylidene fluoride, tetrafluoroethylene-ethylenecopolymers, and polymer compositions having ion conductivity of alkalimetal ions (especially, lithium ions). One of these substances may beused alone or two or more thereof may be used in any combination at anyratio.

The amount of the binder, which is expressed as the proportion of thebinder in the positive electrode active material layer, is usually 0.1%by mass or more, preferably 1% by mass or more, more preferably 1.5% bymass or more. The proportion is also usually 80% by mass or less,preferably 60% by mass or less, still more preferably 40% by mass orless, most preferably 10% by mass or less. Too low a proportion of thebinder may fail to sufficiently hold the positive electrode activematerial and cause insufficient mechanical strength of the positiveelectrode, impairing the battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may causereduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, and salts thereof. One ofthese agents may be used alone or two or more thereof may be used in anycombination at any ratio.

The proportion of the thickening agent relative to the active materialis usually 0.1% by mass or higher, preferably 0.2% by mass or higher,more preferably 0.3% by mass or higher, while usually 5% by mass orlower, preferably 3% by mass or lower, more preferably 2% by mass orlower. The thickening agent at a proportion lower than the above rangemay cause significantly poor easiness of application. The thickeningagent at a proportion higher than the above range may cause a lowproportion of the active material in the positive electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the positive electrode active materials.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite, including natural graphite andartificial graphite, carbon black, including acetylene black, andamorphous carbon, including needle coke. One of these materials may beused alone or two or more thereof may be used in any combination at anyratio. The conductive material is used in an amount of usually 0.01% bymass or more, preferably 0.1% by mass or more, more preferably 1% bymass or more, while usually 50% by mass or less, preferably 30% by massor less, more preferably 15% by mass or less, in the positive electrodeactive material layer. The conductive material in an amount less thanthe above range may cause insufficient conductivity. In contrast,conductive material in an amount more than the above range may cause alow battery capacity.

In order to further improve the high-temperature storagecharacteristics, the positive electrode current collector is preferablyformed from a valve metal or an alloy thereof. Examples of the valvemetal include aluminum, titanium, tantalum, and chromium. The positiveelectrode current collector is more preferably formed from aluminum oran alloy of aluminum.

In order to further improve the high-temperature storage characteristicsof the lithium ion secondary battery, a portion in contact with theelectrolyte solution among portions electrically coupled with thepositive electrode current collector is also preferably formed from avalve metal or an alloy thereof. In particular, the external case of thebattery and a portion that is electrically coupled with the positiveelectrode current collector and is in contact with the non-aqueouselectrolyte solution among components accommodated in the external caseof the battery, such as leads and a safety valve, are preferably formedfrom a valve metal or an alloy thereof. Stainless steel coated with avalve metal or an alloy thereof may also be used.

The positive electrode may be produced by the aforementioned method. Anexample of the production method is a method in which the positiveelectrode active material is mixed with the aforementioned binder,thickening agent, conductive material, solvent, and other components toform a slurry-like positive electrode mixture, and then this mixture isapplied to a positive electrode current collector, dried, and pressed soas to be densified.

The structure of the negative electrode is as described above.

The electric double-layer capacitor may include a positive electrode, anegative electrode, and the aforementioned electrolyte solution.

At least one selected from the positive electrode and the negativeelectrode is a polarizable electrode in the electric double-layercapacitor. Examples of the polarizable electrode and a non-polarizableelectrode include the following electrodes specifically disclosed in JPH09-7896 A.

The polarizable electrode mainly containing activated carbon to be usedin the invention preferably contains inactivated carbon having a largespecific surface area and a conductive material, such as carbon black,providing electronic conductivity. The polarizable electrode may beformed by a variety of methods. For example, a polarizable electrodeincluding activated carbon and carbon black can be produced by mixingactivated carbon powder, carbon black, and phenolic resin, press-moldingthe mixture, and then sintering and activating the mixture in an inertgas atmosphere and water vapor atmosphere. Preferably, this polarizableelectrode is bonded to a current collector using a conductive adhesive,for example.

Alternatively, a polarizable electrode can also be formed by kneadingactivated carbon powder, carbon black, and a binder in the presence ofan alcohol, forming the mixture into a sheet, and then drying the sheet.The binder to be used may be polytetrafluoroethylene, for example.Alternatively, a polarizable electrode integrated with a currentcollector can be produced by mixing activated carbon powder, carbonblack, a binder, and a solvent to form slurry, applying this slurry tometal foil of a current collector, and then drying the slurry.

The electric double-layer capacitor may have polarizable electrodesmainly containing activated carbon as the respective electrodes. Still,the electric double-layer capacitor may have a structure in which anon-polarizable electrode is used on one side. Examples of such astructure include a structure in which a positive electrode mainlycontaining an electrode active material such as a metal oxide iscombined with a polarizable negative electrode mainly containingactivated carbon; and a structure in which a negative electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions or a negative electrode of lithium metal or lithium alloyis combined with a polarizable positive electrode mainly containingactivated carbon.

In place of or in combination with activated carbon, any carbonaceousmaterial may be used, such as carbon black, graphite, expanded graphite,porous carbon, carbon nanotube, carbon nanohorn, and Ketjenblack.

The non-polarizable electrode is preferably an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions, with this carbon material made to occlude lithium ions inadvance. In this case, the electrolyte used is a lithium salt. Theelectric double-layer capacitor having such a structure can achieve amuch higher withstand voltage exceeding 4 V.

The solvent used in preparation of the slurry in production ofelectrodes is preferably one that dissolves a binder. In accordance withthe type of a binder, the solvent is appropriately selected fromN-methylpyrrolidone, dimethyl formamide, toluene, xylene, isophorone,methyl ethyl ketone, ethyl acetate, methyl acetate, dimethyl phthalate,ethanol, methanol, butanol, and water.

Examples of the activated carbon used for the polarizable electrodeinclude phenol resin-type activated carbon, coconut shell-type activatedcarbon, and petroleum coke-type activated carbon. In order to achieve alarge capacity, petroleum coke-type activated carbon or phenolresin-type activated carbon is preferably used. Examples of methods ofactivating the activated carbon include steam activation and molten KOHactivation. In order to achieve a larger capacity, activated carbonprepared by molten KOH activation is preferably used.

Preferred examples of the conductive agent used for the polarizableelectrode include carbon black, Ketjenblack, acetylene black, naturalgraphite, artificial graphite, metal fiber, conductive titanium oxide,and ruthenium oxide. In order to achieve good conductivity (i.e., lowinternal resistance), and because too large an amount thereof may leadto a decreased capacity of the product, the amount of the conductiveagent such as carbon black used for the polarizable electrode ispreferably 1 to 50% by mass in the sum of the amounts of the activatedcarbon and the conductive agent.

In order to provide an electric double-layer capacitor having a largecapacity and low internal resistance, the activated carbon used for thepolarizable electrode preferably has an average particle size of 20 μmor smaller and a specific surface area of 1500 to 3000 m²/g. Preferredexamples of the carbon material for providing an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions include natural graphite, artificial graphite, graphitizedmesocarbon microsphere, graphitized whisker, vapor-grown carbon fiber,sintered furfuryl alcohol resin, and sintered novolak resin.

The current collector may be any chemically and electrochemicallycorrosion-resistant one. Preferred examples of the current collectorused for the polarizable electrode mainly containing activated carboninclude stainless steel, aluminum, titanium, and tantalum. Particularlypreferred materials in terms of the characteristics and cost of theresulting electric double-layer capacitor are stainless steel andaluminum. Preferred examples of the current collector used for theelectrode mainly containing a carbon material that can reversiblyocclude and release lithium ions include stainless steel, copper, andnickel.

Examples of methods of allowing the carbon material that can reversiblyocclude and release lithium ions to occlude lithium ions in advanceinclude: (1) a method of mixing powdery lithium to a carbon materialthat can reversibly occlude and release lithium ions; (2) a method ofplacing lithium foil on an electrode including a carbon material thatcan reversibly occlude and release lithium ions and a binder so as tobring the lithium foil to be in electrical contact with the electrode,immersing this electrode in an electrolyte solution containing a lithiumsalt dissolved therein so as to ionize the lithium, and allowing thecarbon material to take in the lithium ions; and (3) a method of placingan electrode including a carbon material that can reversibly occlude andrelease lithium ions and a binder on the minus side and placing alithium metal on the plus side, immersing the electrodes in anon-aqueous electrolyte solution containing a lithium salt as anelectrolyte, and supplying a current so that the carbon material isallowed to electrochemically take in the ionized lithium.

Examples of known electric double-layer capacitors include woundelectric double-layer capacitors, laminated electric double-layercapacitors, and coin-type electric double-layer capacitors. The electricdouble-layer capacitor may also be any of these types.

For example, a wound electric double-layer capacitor may be assembled asfollows. A positive electrode and a negative electrode each of whichincludes a laminate (electrode) of a current collector and an electrodelayer are wound with a separator in between to provide a wound element.This wound element is put into a case made of aluminum, for example. Thecase is filled with an electrolyte solution, preferably a non-aqueouselectrolyte solution, and then sealed with a rubber sealant.

A separator formed from a conventionally known material and having aconventionally known structure may be used. Examples thereof includepolyethylene porous membranes, polytetrafluoroethylene, and nonwovenfabric of polypropylene fiber, glass fiber, or cellulose fiber.

In accordance with any known method, the electric double-layer capacitormay be prepared in the form of a laminated electric double-layercapacitor in which sheet-like positive and negative electrodes arestacked with an electrolyte solution and a separator in between or acoin-type electric double-layer capacitor in which positive and negativeelectrodes are fixed in a coin shape by a gasket with an electrolytesolution and a separator in between.

The electrolyte solution of the invention is useful as an electrolytesolution for large-size lithium ion secondary batteries for hybridvehicles or distributed generation, and for electric double-layercapacitors.

EXAMPLES

The invention is described with reference to examples, but the inventionis not intended to be limited by these examples.

Synthesis Example 1 <Production of 4-allyloxy-1,3-dioxolan-2-one>Additive (I)

Vinylene carbonate (8.6 g, 100 mmol) and triethyl amine (1.0 g, 10 mmol)were mixed. The system was subjected to nitrogen purging, dropwiseaddition of allyl alcohol (5.8 g, 100 mmol) at 0° C., and then stirringfor one hour at room temperature. After the reaction, neutralizationwith 1N hydrochloric acid and washing with saturated sodium bicarbonatewater were sequentially performed. The organic layer was dried andcondensed, whereby 13.1 g of the target product was obtained.

Synthesis Example 2 <Production of 4-(2-propynyloxy)-1,3-dioxolan-2-one>Additive (II)

Vinylene carbonate (8.6 g, 100 mmol) and triethyl amine (1.0 g, 10 mmol)were mixed. The system was subjected to nitrogen purging, dropwiseaddition of propargyl alcohol (5.6 g, 100 mmol) at 0° C., and thenstirring for one hour at room temperature. After the reaction,neutralization with 1N hydrochloric acid and washing with saturatedsodium bicarbonate water were sequentially performed. The organic layerwas dried and condensed, whereby 12.2 g of the target product wasobtained.

Synthesis Example 3 <Production of 4-ethoxy-1,3-dioxolan-2-one> Additive(III)

Vinylene carbonate (8.6 g, 100 mmol) and triethyl amine (1.0 g, 10 mmol)were mixed. The system was subjected to nitrogen purging, dropwiseaddition of ethanol (4.6 g, 100 mmol) at 0° C., and then stirring forone hour at room temperature. After the reaction, neutralization with 1Nhydrochloric acid and washing with saturated sodium bicarbonate waterwere sequentially performed. The organic layer was dried and condensed,whereby 11.9 g of the target product was obtained.

Synthesis Example 4 <Production of4-(2-fluoroallyloxy)-1,3-dioxolan-2-one> Additive (V)

Vinylene carbonate (86 mg, 1 mmol) and triethyl amine (10 mg, 0.1 mmol)were mixed. The system was subjected to nitrogen purging, and thendropwise addition of 2-fluoropropene 1-ol (76 mg, 1 mmol) at 0° C. Theresulting solution was warmed to room temperature and then stirred forone hour, whereby 146 mg of the target product was obtained.

Synthesis Example 5 <Production of4-(4,4,4-trifluoro-2-butenoxy)-1,3-dioxolan-2-one> Additive (VI)

Vinylene carbonate (86 mg, 1 mmol) and triethyl amine (10 mg, 0.1 mmol)were mixed. The system was subjected to nitrogen purging, and thendropwise addition of 4,4,4-trifluoro-2-buten-1-ol (126 mg, 1 mmol) at 0°C. The resulting solution was warmed to room temperature and thenstirred for one hour, whereby 195 mg of the target product was obtained.

Synthesis Example 6 <Production of4-(3-trimethylsilyl)2-propynyloxy)-1,3-dioxolan-2-one> Additive (VII)

Vinylene carbonate (8.6 g, 100 mmol) and triethyl amine (1.0 g, 10 mmol)were mixed. The system was subjected to nitrogen purging, dropwiseaddition of trimethylsilyl propargyl alcohol (12.8 g, 100 mmol) at 0°C., and then stirring for one hour at room temperature. After thereaction, neutralization with 1N hydrochloric acid and washing withsaturated sodium bicarbonate water were sequentially performed. Theorganic layer was dried and condensed, whereby 18.2 g of the targetproduct was obtained.

Synthesis Example 7 <Production of4-(2-fluoroallyloxy)-4,5-dimethyl-1,3-dioxolan-2-one> Additive (VIII)

4,5-Dimethyl vinylene carbonate (114 mg, 1 mmol) and triethyl amine (10mg, 0.1 mmol) were mixed. The system was subjected to nitrogen purging,and then dropwise addition of 2-fluoropropene 1-ol (76 mg, 1 mmol) at 0°C. The resulting solution was warmed to room temperature and thenstirred for one hour, whereby 148 mg of the target product was obtained.

Examples and Comparative Examples Shown in Table 1 [Characteristics of4.2 V Lithium Ion Secondary Battery] (Preparation of ElectrolyteSolution)

A solvent mixture of EC (ethylene carbonate), EMC (ethyl methylcarbonate), and DMC (dimethyl carbonate) (volume ratio:EC/EMC/DMC=30/30/40) was prepared as a solvent for a non-aqueouselectrolyte solution. LiPF₆ as a support electrolyte solution wasdissolved therein at a concentration of 1.0 mol/L. To this referenceelectrolyte solution was added one of the additives prepared inSynthesis Examples 1 to 7 according to Table 1, whereby a non-aqueouselectrolyte solution was obtained.

(Production of Aluminum Laminate-Type Lithium Ion Secondary Battery)[Production of Positive Electrode]

First, 90% by mass of Li(Ni_(1/3)Mn₁₈₃Co_(1/3)) O₂ (LNMO) serving as apositive electrode active material, 5% by mass of acetylene blackserving as a conductive material, and 5% by mass of polyvinylidenefluoride (PVdF) serving as a binding agent were mixed in aN-methylpyrrolidone solvent to form slurry. The resulting slurry wasapplied to one surface of 15-μm-thick aluminum foil with a conductiveaid applied thereto in advance, and dried. The workpiece was thenroll-pressed using a press and cut to provide a piece including anactive material layer having a width of 50 mm and a length of 30 mm andan uncoated portion having a width of 5 mm and a length of 9 mm. Thispiece was used as a positive electrode.

[Production of Negative Electrode]

First, 98 parts by mass of a carbonaceous material (graphite) was mixedwith 1 part by mass of an aqueous dispersion of sodium carboxymethylcellulose (concentration of sodium carboxymethyl cellulose: 1% by mass)and 1 part by mass of an aqueous dispersion of styrene-butadiene rubber(concentration of styrene-butadiene rubber: 50% by mass) respectivelyserving as a thickening agent and a binder. The components were mixedusing a disperser to form slurry. The resulting slurry was applied to10-μm-thick copper foil and dried. The workpiece was rolled using apress and cut to provide a piece including an active material layerhaving a width of 52 mm and a length of 32 mm and an uncoated portionhaving a width of 5 mm and a length of 9 mm. This piece was used as anegative electrode.

[Production of Aluminum Laminate Cell]

The above positive electrode and negative electrode were placed to faceeach other with a 20-μm-thick porous polyethylene film (separator) inbetween. The non-aqueous electrolyte solution prepared above was filledthereinto and the non-aqueous electrolyte solution was made tosufficiently permeate into the components such as the separator. Theworkpiece was then sealed, pre-charged, and aged, whereby a lithium ionsecondary battery was produced.

(Determination of Battery Characteristics) [Cycle Capacity Retention]

The lithium ion secondary battery in a state of being sandwiched andpressurized between plates was subjected to constant current/constantvoltage charge (hereinafter, referred to as CC/CV charge) (0.1 C cutoff) to 4.2 V at a current corresponding to 1 C and at 60° C., and thendischarged to 3 V at a constant current of 1 C. This was counted as onecycle, and the discharge capacity at the third cycle was used todetermine the initial discharge capacity. Here, 1 C means the currentvalue at which the reference capacity of the battery is discharged inone hour. For example, 0.2 C means 1/5 of this current value. The cycleswere again performed, and the discharge capacity after the 150th cyclewas determined. The ratio of the discharge capacity after the 150thcycle to the initial discharge capacity was determined. This value wasdefined as the cycle capacity retention (%). The results are shown inTable 1.

(Discharge capacity after 150th cycle)/(Initial dischargecapacity)×100=Capacity retention (%)

[Amount of Gas Generated]

The volume of the lithium ion secondary battery was measured afterproduction and after 150 cycles. The amount (ml) of gas generated wasdetermined from the following formula. The results are shown in Table 1.

(Volume after 150 cycles)−(Initial volume)=(Amount of gas generated(ml))

[Evaluation of IV Resistance]

The lithium secondary battery after the evaluation of initial dischargecapacity was charged at a constant current of 1 C and at 25° C. so as tohave a capacity that is half the initial discharge capacity. The batterywas then discharged at 2.0 C and at 25° C., and the voltage at the 10thsecond was measured. The resistance was calculated from the voltage dropduring discharge, which was defined as the IV resistance. The resultsare shown in Table 1.

TABLE 1 Additive Cycle capacity Amount of gas IV Amount retentiongenerated resistance Type (% by mass) (%) (ml) (Ω) Example 1 (I) 1.0 941.3 2.3 Example 2 (I) 0.3 91 1.4 2.5 Example 3 (I) 0.05 89 1.6 2.6Example 4 (I) 3.5 90 1.5 2.3 Example 5 (I) 8.0 87 1.4 2.4 Example 6 (II)1.0 90 1.1 2.5 Example 7 (II) 0.2 88 1.3 2.4 Example 8 (III) 1.0 87 1.82.6 Example 9 (V) 1.0 90 1.6 1.9 Example 10 (VI) 1.0 88 1.2 2.0 Example11 (VII) 1.0 89 1.7 2.1 Example 12 (VIII) 1.0 99 1.5 2.2 Example 13 (I)1.0 92 1.9 2.7 VC 1.0 Example 14 (I) 1.0 93 2.1 2.8 FEC 1.0 Example 15(II) 1.0 91 1.7 2.7 VC 1.0 Example 16 (II) 1.0 92 2.1 2.5 FEC 1.0Example 17 (II) 1.0 92 1.9 2.7 LiBOB 1.0 Example 18 (II) 1.0 91 1.6 1.9LiPO2F2 1.0 Example 19 (II) 1.0 91 1.9 2.0 FSO3Li 1.0 Example 20 (II)1.0 91 1.8 2.1 C2H5OSO3Li 1.0 Example 21 (III) 1.0 89 1.9 2.4 VC 1.0Example 22 (III) 1.0 91 1.5 2.6 FEC 1.0 Example 23 (V) 1.0 91 1.7 2.1 VC1.0 Example 24 (V) 1.0 92 2.0 2.0 FEC 1.0 Example 25 (V) 1.0 91 1.8 2.2LiBOB 1.0 Example 26 (V) 1.0 92 1.6 1.7 LiPO2F2 1.0 Example 27 (V) 1.092 1.7 1.8 FSO3Li 1.0 Example 28 (V) 1.0 91 1.7 1.8 C2H5OSO3Li 1.0Example 29 (VI) 1.0 90 1.4 2.3 VC 1.0 Example 30 (VI) 1.0 90 1.7 2.2 FEC1.0 Comparative — — 73 2.5 2.4 Example 1 Comparative (IV) 1.0 79 2.9 2.7Example 2 Comparative VC 1.0 80 2.6 3.0 Example 3 Comparative FEC 1.0 793.0 2.7 Example 4

The compounds shown in Table 1 are as follows.

VC: vinylene carbonateFEC: fluoroethylene carbonate

Examples and Comparative Examples Shown in Table 2 [Characteristics of4.9 V Lithium Ion Secondary Battery] (Preparation of ElectrolyteSolution)

The components were mixed at a composition shown in Table 2. To thismixture LiPFE was added at a concentration of 1.0 mol/L, whereby anon-aqueous electrolyte solution was obtained.

(Production of Aluminum Laminate-Type Lithium Ion Secondary Battery)[Production of Positive Electrode]

First, LiNi_(0.5)Mn_(1.5)O₄ serving as a positive electrode activematerial, acetylene black serving as a conductive material, andpolyvinylidene fluoride (PVdF) dispersed in a N-methyl-2-pyrrolidoneserving as a binding agent were mixed to form positive electrode mixtureslurry. The positive electrode active material, conductive material, andbinding agent gave a solid content ratio of 92/3/5 (ratio by % by mass).This slurry was applied to one surface of 15-μm-thick aluminum foil anddried. The workpiece was then roll-pressed using a press and cut toprovide a piece including an active material layer having a width of 50mm and a length of 30 mm and an uncoated portion having a width of 5 mmand a length of 9 mm. This piece was used as a positive electrode.

[Production of Negative Electrode]

A negative electrode was produced in the same manner as in the examplesshown in Table 1.

[Production of Aluminum Laminate Cell]

An aluminum laminate cell was produced in the same manner as in theexamples shown in Table 1.

(Determination of Battery Characteristics) [Cycle Capacity Retention]

The lithium ion secondary battery in a state of being sandwiched andpressurized between plates was subjected to constant current/constantvoltage charge (hereinafter, referred to as CC/CV charge) (0.1 C cutoff) to 4.9 V at a current corresponding to 1 C and at 60° C., and thendischarged to 3 V at a constant current of 1 C. This was counted as onecycle, and the discharge capacity at the third cycle was used todetermine the initial discharge capacity. The cycles were againperformed, and the discharge capacity after the 200th cycle wasdetermined. The ratio of the discharge capacity after the 200th cycle tothe initial discharge capacity was determined. This value was defined asthe cycle capacity retention (%). The results are shown in Table 2.

(Discharge capacity after 200th cycle)/(Initial dischargecapacity)×100=Capacity retention (%)

[Amount of Gas]

The volume of the lithium ion secondary battery was measured afterproduction and after 200 cycles. The amount (ml) of gas generated wasdetermined from the following formula. The results are shown in Table 2.

(Volume after 200 cycles)−(Initial volume)=(Amount of gas generated(ml))

[Evaluation of IV Resistance]

The battery after the evaluation of initial discharge capacity wascharged at a constant current of 1 C and at 25° C. so as to have acapacity that is half the initial discharge capacity. The battery wasthen discharged at 2.0 C and at 25° C., and the voltage at the 10thsecond was measured. The resistance was calculated from the voltage dropduring discharge, which was defined as the IV resistance. The resultsare shown in Table 2.

TABLE 2 Solvent Additive Cycle Amount of Amount Amount Amount capacitygas IV (% by (% by (% by retention generated resistance Type volume)Type volume) Type mass) (%) (ml) (Ω) Example 31 (1-1) 30 (2-1) 70 (I)1.0 88 3.3 3.3 Example 32 (1-1) 30 (2-1) 70 (I) 0.3 84 3.4 3.5 Example33 (1-1) 30 (2-1) 70 (I) 0.05 83 3.5 4.3 Example 34 (1-1) 30 (2-1) 70(I) 3.5 82 3.4 3.8 Example 35 (1-1) 30 (2-1) 70 (I) 8.0 79 3.7 3.8Example 36 (1-1) 30 (2-1) 70 (II) 1.0 82 3.9 3.4 Example 37 (1-1) 30(2-1) 70 (II) 0.2 76 4.2 3.1 Example 38 (1-1) 30 (2-1) 70 (III) 1.0 754.4 3.3 Example 39 (1-1) 30 (2-1) 70 (III) 0.2 73 3.8 3.2 Example 40(1-1) 30 (2-1) 70 (V) 1.0 87 4.4 3.0 Example 41 (1-1) 30 (2-1) 70 (V)0.2 83 4.1 2.8 Example 42 (1-1) 30 (2-1) 70 (VI) 1.0 82 3.7 3.1 Example43 (1-1) 30 (2-1) 70 (VI) 0.2 80 3.6 3.0 Example 44 (1-1) 30 (2-2) 70(I) 1.0 85 3.5 2.8 Example 45 (1-2) 30 (2-1) 70 (I) 1.0 75 2.1 3.0Example 46 (1-2) 30 (2-2) 70 (I) 1.0 76 2.5 2.6 Example 47 (1-1) 30(2-1) 70 (I) 12 70 4 4.2 Comparative (1-1) 30 (2-1) 70 — — 38 7.2 6.6Example 5

The compounds shown in Table 2 are as follows.

(I) to (III) and (V) and (VI): the same as mentioned in Table 1

Examples and Comparative Examples Shown in Table 3 [Characteristics of4.35 V Lithium Ion Secondary Battery] (Preparation of ElectrolyteSolution)

The components were mixed at a composition shown in Table 3. To thismixture LiPF₆ was added at a concentration of 1.2 mol/L, whereby anon-aqueous electrolyte solution was obtained.

(Production of Aluminum Laminate-Type Lithium Ion Secondary Battery)[Production of Positive Electrode]

First, 90% by mass of Li(Ni_(1/3)Mn_(1/3)Co_(1/3)) O₂ (LNMO) serving asa positive electrode active material, 5% by mass of acetylene blackserving as a conductive material, and 5% by mass of polyvinylidenefluoride (PVdF) serving as a binding agent were mixed in aN-methylpyrrolidone solvent to form slurry. The resulting slurry wasapplied to one surface of 15-μm-thick aluminum foil with a conductiveaid applied thereto in advance, and dried. The workpiece was thenroll-pressed using a press and cut to provide a piece including anactive material layer having a width of 50 mm and a length of 30 mm andan uncoated portion having a width of 5 mm and a length of 9 mm. Thispiece was used as a positive electrode.

[Production of Negative Electrode]

A negative electrode was produced in the same manner as in the examplesshown in Table 1.

[Production of Aluminum Laminate Cell]

An aluminum laminate cell was produced in the same manner as in theexamples shown in Table 1.

(Determination of Battery Characteristics) [Cycle Capacity Retention]

The lithium ion secondary battery in a state of being sandwiched andpressurized between plates was subjected to constant current/constantvoltage charge (hereinafter, referred to as CC/CV charge) (0.1 C cutoff) to 4.35 V at a current corresponding to 1 C and at 45° C., and thendischarged to 3 V at a constant current of 1 C. This was counted as onecycle, and the discharge capacity at the third cycle was used todetermine the initial discharge capacity. The cycles were againperformed, and the discharge capacity after the 200th cycle wasdetermined. The ratio of the discharge capacity after the 200th cycle tothe initial discharge capacity was determined. This value was defined asthe cycle capacity retention (%). The results are shown in Table 3.

(Discharge capacity after 200th cycle)/(Initial dischargecapacity)×100=Capacity retention (%)

[Amount of Gas]

The volume of the lithium ion secondary battery was measured afterproduction and after 200 cycles. The amount (ml) of gas generated wasdetermined from the following formula. The results are shown in Table 3.

(Volume after 200 cycles)−(Initial volume)=(Amount of gas generated(ml))

[Evaluation of IV Resistance]

The battery after the evaluation of initial discharge capacity wascharged at a constant current of 1 C and at 25° C. so as to have acapacity that is half the initial discharge capacity. The battery wasthen discharged at 2.0 C and at 25° C., and the voltage at the 10thsecond was measured. The resistance was calculated from the voltage dropduring discharge, which was defined as the IV resistance. The resultsare shown in Table 3.

TABLE 3 Solvent Additive Cycle Amount of Amount Amount Amount capacitygas IV (% by (% by (% by retention generated resistance Type volume)Type volume) Type mass) (%) (ml) (Ω) Example 48 (1-1) 30 (2-2) 70 (I)1.0 92 2.5 2.6 Example 49 (1-1) 30 (2-3) 70 (I) 1.0 91 2.4 2.6 Example50 (1-1) 30 (2-4) 70 (I) 1.0 90 2.6 2.8 Example 51 (1-1) 30 (2-2) 70(II) 1.0 93 2.6 2.7 Example 52 (1-1) 30 (2-3) 70 (II) 1.0 92 2.5 2.7Example 53 (1-1) 30 (2-4) 70 (II) 1.0 92 2.6 2.8 Example 54 (1-1) 30(2-2) 70 (III) 1.0 89 2.9 2.5 Example 55 (1-1) 30 (2-2) 70 (V) 1.0 912.6 2.4 Example 56 (1-1) 30 (2-2) 70 (VI) 1.0 91 2.7 2.4 Example 57(1-1) 30 (2-2) 70 (VII) 1.0 92 2.9 2.3 Example 58 (1-1) 30 (2-2) 70(VIII) 1.0 90 2.7 2.5 Comparative (1-1) 30 (2-2) 70 — — 80 4.4 3.7Example 6 Comparative (1-1) 30 (2-3) 70 — — 80 4.8 3.8 Example 7Comparative (1-1) 30 (2-4) 70 — — 79 4.8 3.5 Example 8

The compounds shown in Table 3 are as follows.

(I) to (III) and (V) to (VIII): the same as mentioned in Table 1

1. An electrolyte solution comprising a compound (1) represented by thefollowing formula (1):

wherein R¹⁰¹ and R¹⁰² are the same as or different from each other andare each a hydrogen atom, a fluorine atom, or an alkyl group optionallycontaining a fluorine atom; and R¹⁰³ is an alkyl group or an organicgroup containing an unsaturated carbon-carbon bond.
 2. The electrolytesolution according to claim 1, wherein R¹⁰³ in the formula (1) is anorganic group containing an unsaturated carbon-carbon bond.
 3. Theelectrolyte solution according to claim 1, wherein the compound (1)accounts for 0.001 to 10% by mass of the electrolyte solution.
 4. Theelectrolyte solution according to claim 1, further comprising anelectrolyte salt.
 5. An electrochemical device comprising theelectrolyte solution according to claim
 1. 6. A lithium ion secondarybattery comprising the electrolyte solution according to claim
 1. 7. Amodule comprising the electrochemical device according to claim
 5. 8. Amodule comprising the lithium ion secondary battery according to claim6.